I'm having a very similar issue, running Linux Mint 19.1. The issue has persisted from at least 4.15, I'm currently running 5.0.1 and the issue remains. 

Here is the latest syslog of the error:

[37258.615599] gmc_v9_0_process_interrupt: 10 callbacks suppressed
[37258.615608] amdgpu 0000:06:00.0: [gfxhub] VMC page fault (src_id:0 ring:24 vmid:3 pasid:32768, for process Xorg pid 1287 thread Xorg:cs0 pid 1317)
[37258.615615] amdgpu 0000:06:00.0:   in page starting at address 0x0000800107805000 from 27
[37258.615619] amdgpu 0000:06:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00301031
[37258.615629] amdgpu 0000:06:00.0: [gfxhub] VMC page fault (src_id:0 ring:24 vmid:3 pasid:32768, for process Xorg pid 1287 thread Xorg:cs0 pid 1317)
[37258.615633] amdgpu 0000:06:00.0:   in page starting at address 0x0000800107807000 from 27
[37258.615636] amdgpu 0000:06:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00000000
[37258.615645] amdgpu 0000:06:00.0: [gfxhub] VMC page fault (src_id:0 ring:24 vmid:3 pasid:32768, for process Xorg pid 1287 thread Xorg:cs0 pid 1317)
[37258.615648] amdgpu 0000:06:00.0:   in page starting at address 0x0000800107801000 from 27
[37258.615651] amdgpu 0000:06:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00000000
[37258.615660] amdgpu 0000:06:00.0: [gfxhub] VMC page fault (src_id:0 ring:24 vmid:3 pasid:32768, for process Xorg pid 1287 thread Xorg:cs0 pid 1317)
[37258.615663] amdgpu 0000:06:00.0:   in page starting at address 0x0000800107803000 from 27
[37258.615666] amdgpu 0000:06:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00000000
[37258.615675] amdgpu 0000:06:00.0: [gfxhub] VMC page fault (src_id:0 ring:24 vmid:3 pasid:32768, for process Xorg pid 1287 thread Xorg:cs0 pid 1317)
[37258.615678] amdgpu 0000:06:00.0:   in page starting at address 0x0000800107809000 from 27
[37258.615681] amdgpu 0000:06:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00000000
[37258.615689] amdgpu 0000:06:00.0: [gfxhub] VMC page fault (src_id:0 ring:24 vmid:3 pasid:32768, for process Xorg pid 1287 thread Xorg:cs0 pid 1317)
[37258.615692] amdgpu 0000:06:00.0:   in page starting at address 0x000080010780b000 from 27
[37258.615695] amdgpu 0000:06:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00000000
[37258.615704] amdgpu 0000:06:00.0: [gfxhub] VMC page fault (src_id:0 ring:24 vmid:3 pasid:32768, for process Xorg pid 1287 thread Xorg:cs0 pid 1317)
[37258.615707] amdgpu 0000:06:00.0:   in page starting at address 0x0000800107805000 from 27
[37258.615710] amdgpu 0000:06:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00000000
[37258.615740] amdgpu 0000:06:00.0: [gfxhub] VMC page fault (src_id:0 ring:24 vmid:3 pasid:32768, for process Xorg pid 1287 thread Xorg:cs0 pid 1317)
[37258.615743] amdgpu 0000:06:00.0:   in page starting at address 0x0000800107807000 from 27
[37258.615746] amdgpu 0000:06:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00000000
[37258.615756] amdgpu 0000:06:00.0: [gfxhub] VMC page fault (src_id:0 ring:24 vmid:3 pasid:32768, for process Xorg pid 1287 thread Xorg:cs0 pid 1317)
[37258.615759] amdgpu 0000:06:00.0:   in page starting at address 0x0000800107801000 from 27
[37258.615762] amdgpu 0000:06:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00000000
[37258.615771] amdgpu 0000:06:00.0: [gfxhub] VMC page fault (src_id:0 ring:24 vmid:3 pasid:32768, for process Xorg pid 1287 thread Xorg:cs0 pid 1317)
[37258.615774] amdgpu 0000:06:00.0:   in page starting at address 0x0000800107803000 from 27
[37258.615777] amdgpu 0000:06:00.0: VM_L2_PROTECTION_FAULT_STATUS:0x00000000
[37268.712339] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37268.712387] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37268.712389] [drm] GPU recovery disabled.
[37278.952537] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37278.952624] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37278.952628] [drm] GPU recovery disabled.
[37289.192390] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37289.192478] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37289.192481] [drm] GPU recovery disabled.
[37299.432447] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37299.432534] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37299.432538] [drm] GPU recovery disabled.
[37309.676431] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37309.676518] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37309.676522] [drm] GPU recovery disabled.
[37319.912444] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37319.912536] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37319.912541] [drm] GPU recovery disabled.
[37330.156619] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37330.156706] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37330.156710] [drm] GPU recovery disabled.
[37340.392424] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37340.392511] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37340.392515] [drm] GPU recovery disabled.
[37350.632424] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37350.632511] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37350.632514] [drm] GPU recovery disabled.
[37360.872417] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37360.872508] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37360.872511] [drm] GPU recovery disabled.
[37371.112436] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37371.112523] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37371.112527] [drm] GPU recovery disabled.
[37381.352427] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37381.352514] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37381.352517] [drm] GPU recovery disabled.
[37391.592410] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37391.592497] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37391.592500] [drm] GPU recovery disabled.
[37401.836426] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37401.836513] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37401.836517] [drm] GPU recovery disabled.
[37412.072433] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37412.072520] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37412.072524] [drm] GPU recovery disabled.
[37422.312442] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37422.312528] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37422.312532] [drm] GPU recovery disabled.
[37432.552428] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37432.552515] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37432.552519] [drm] GPU recovery disabled.
[37442.792418] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37442.792506] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37442.792510] [drm] GPU recovery disabled.
[37453.032397] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37453.032483] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37453.032487] [drm] GPU recovery disabled.
[37463.272534] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37463.272621] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37463.272624] [drm] GPU recovery disabled.
[37473.512589] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37473.512676] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37473.512680] [drm] GPU recovery disabled.
[37483.752954] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37483.753041] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37483.753044] [drm] GPU recovery disabled.
[37493.992566] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* ring gfx timeout, signaled seq=602475, emitted seq=602478
[37493.992654] [drm:amdgpu_job_timedout [amdgpu]] *ERROR* Process information: process Xorg pid 1287 thread Xorg:cs0 pid 1317
[37493.992657] [drm] GPU recovery disabled.

During this time the laptop continues to operate (plays music and can SSH in), however the display and any input (keyboard / mouse) do not respond. The caps lock light for example does not toggle. The only way to recover is a force reboot by holding the power button.

I'm unable to provide any steps on how to re-create as the issue happens at completely random times when performing different tasks or when leaving the machine idle. 

System specs:
Lenovo ThinkPad A485
AMD Ryzen 7 PRO 2700U with Radeon Vega Mobile Gfx
Linux Mint 19.1
Kernel 5.0.1 (installed via ukuu)