drm/amdgpu: add gc powerbrake support (v2)

add GC power brake feature support for Aldebaran.

v2: squash in fixes (Alex)

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>