drm/amd/amdgpu: Introduction of SI registers (v2)

This introduces the SI registers in the amdgpu
driver style.

v2: squash duplicates fix

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>