drm/amdgpu: add tracking for the enablement of SCPM

Add parmeter to shows whether SCPM feature is enabled or not, and
whether is valid.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>