I get a simliar issue to you on my A10-8780P, although on 16.04 I can't even get it to boot without nomodeset as the blackscreen is as far as it gets. Not even sure to know where to begin getting more info on that but:

jenni@hp-grey:~/Desktop$ lspci -vvnn | grep -i AMD
00:00.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1576]
00:00.2 IOMMU [0806]: Advanced Micro Devices, Inc. [AMD] Device [1022:1577]
00:01.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Carrizo [1002:9874] (rev c4) (prog-if 00 [VGA controller])
00:01.1 Audio device [0403]: Advanced Micro Devices, Inc. [AMD/ATI] Kabini HDMI/DP Audio [1002:9840]
00:02.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:157b]
00:02.2 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:157c] (prog-if 00 [Normal decode])
00:02.3 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:157c] (prog-if 00 [Normal decode])
00:02.4 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:157c] (prog-if 00 [Normal decode])
00:03.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:157b]
00:03.1 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:157c] (prog-if 00 [Normal decode])
00:08.0 Encryption controller [1080]: Advanced Micro Devices, Inc. [AMD] Device [1022:1578]
00:09.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:157d]
00:09.2 Audio device [0403]: Advanced Micro Devices, Inc. [AMD] Device [1022:157a]
00:10.0 USB controller [0c03]: Advanced Micro Devices, Inc. [AMD] FCH USB XHCI Controller [1022:7914] (rev 20) (prog-if 30 [XHCI])
00:11.0 SATA controller [0106]: Advanced Micro Devices, Inc. [AMD] FCH SATA Controller [AHCI mode] [1022:7901] (rev 49) (prog-if 01 [AHCI 1.0])
00:12.0 USB controller [0c03]: Advanced Micro Devices, Inc. [AMD] FCH USB EHCI Controller [1022:7908] (rev 49) (prog-if 20 [EHCI])
	Subsystem: Advanced Micro Devices, Inc. [AMD] FCH USB EHCI Controller [1022:7908]
00:14.0 SMBus [0c05]: Advanced Micro Devices, Inc. [AMD] FCH SMBus Controller [1022:790b] (rev 4a)
	Subsystem: Advanced Micro Devices, Inc. [AMD] FCH SMBus Controller [1022:790b]
00:14.3 ISA bridge [0601]: Advanced Micro Devices, Inc. [AMD] FCH LPC Bridge [1022:790e] (rev 11)
	Subsystem: Advanced Micro Devices, Inc. [AMD] FCH LPC Bridge [1022:790e]
00:18.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1570]
00:18.1 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1571]
00:18.2 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1572]
00:18.3 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1573]
00:18.4 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1574]
00:18.5 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1575]
04:00.0 Display controller [0380]: Advanced Micro Devices, Inc. [AMD/ATI] Topaz XT [Radeon R7 M260/M265] [1002:6900] (rev ff) (prog-if ff)


It does seem to work with the fglrx driver on 14.04 (although even then I had to boot using nomodeset before installing the fglrx driver as I had a similar black screen issue there)

Are these issues related?

James