Comment 2 for bug 1756324

I'm not entirely sure if this is related, but I'm also having the same error and games that rely on opengl stoped working since (~02/03/2018 - dd-mm-yyyy):

I have an AMD Radeon HD 7850 installed.

It seems to be related to the xorg update or the other ones around (please see attachment):
/var/log/apt/history
xserver-xorg-core:amd64 (2:1.19.6-1ubuntu1, 2:1.19.6-1ubuntu2)

/var/log/kernel
Mar 15 19:21:22 dktp kernel: [ 2.118654] [drm] VGACON disable radeon kernel modesetting.
Mar 15 19:21:22 dktp kernel: [ 2.118687] [drm:radeon_init [radeon]] *ERROR* No UMS support in radeon module!
Mar 15 19:21:22 dktp kernel: [ 2.216849] [drm:amdgpu_init [amdgpu]] *ERROR* VGACON disables amdgpu kernel modesetting.

Making a basic tests, it seems to perform better with software rendering... ?!?

$ glxgears -info
GL_RENDERER = AMD PITCAIRN (DRM 2.50.0 / 4.15.0-11-generic, LLVM 5.0.1)
GL_VERSION = 3.0 Mesa 18.0.0-rc4
GL_VENDOR = X.Org
(...)
VisualID 1064, 0x428
355 frames in 5.0 seconds = 70.940 FPS
300 frames in 5.0 seconds = 59.997 FPS

$ LIBGL_ALWAYS_SOFTWARE=1 glxgears -info
GL_RENDERER = llvmpipe (LLVM 5.0, 128 bits)
GL_VERSION = 3.0 Mesa 18.0.0-rc4
GL_VENDOR = VMware, Inc.
(...)
VisualID 1064, 0x428
6604 frames in 5.0 seconds = 1320.753 FPS
6758 frames in 5.0 seconds = 1351.475 FPS

Further details:
$ xdpyinfo | grep DRI
    DRI2
    DRI3
$ lspci -k | grep -EA3 'VGA|3D|Display'
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Pitcairn PRO [Radeon HD 7850 / R7 265 / R9 270 1024SP]
 Subsystem: PC Partner Limited / Sapphire Technology Radeon HD 7850 2GB GDDR5 DVI-I/DVI-D/HDMI/DP
 Kernel driver in use: radeon
 Kernel modules: radeon, amdgpu

Can anyone help?