drm/amd/display: read VM settings from MMHUB

instead of GC, as after GFX off, GC can be power gated any time

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>