[PATCH] drm/{amdgpu, radeon}: check PS, WS index

[Alexander]

These patches add index checking to parameter / work space reads and writes to
ATOMBIOS bytecode. Theoretically, it would be possible to perform stack-aligned
out-of-bounds read and writes, and since the bytecode is in the BIOS, any     
exploits which could be made with this would persist across reboots. 

This is my first patch to the Linux kernel; I did my best to follow the style 
guidelines, I hope I didn't make too many mistakes!
Tested on a R5 340X; sadly do not have any more modern GPUs to test this with.

Signed-off-by: Alexander Richards <electrodeyt at gmail.com>