drm/radeon: add fault decode function for CIK

Helpful for debugging GPUVM errors as we can see what
hw block and page generated the fault in the log.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/cik.c
Simple merge

diff --cc drivers/gpu/drm/radeon/cikd.h
Simple merge