drm/amd/powerplay: make mmhub pg bit configured by pg_flags

The mmhub pg features enabling should be indicated by pg_flags.

Reported-by: Lijo Lazar <Lijo.Lazar@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>