drm/amd/powerplay: enable athub pg

enable athub pg and the status can be checked in
ATHUB_MISC_CNTL.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>