drm/amdgpu/: add more macro to support offset variant

Add more macro to support offset variant and
simplify macro SOC15_WAIT_ON_RREG.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>