drm/amdgpu: fix coding error of mmhub pg enablement

MMHUB powergating should be disabled on navi12 and enabled on sienna
cichlid.

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>