drm/amdgpu: add read/write function for GC CAC programming

Create a GC_CAC_IND_INDEX/DATA pair of funcitons to program
all the CAC registers

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