drm/amd/amdgpu: Add debugfs support for reading GPRs (v2)

Implemented for SGPRs for GFX v8 initially.

(v2) cleanup minor whitespace and remove sanity check and
     addressing is in dwords not bytes

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>