drm/radeon: add fault decode function for cayman/TN (v2)

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

v2: simplify fault decoding

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>