drm/amdgpu: disable MMHUB power gating on raven

MMHUB power gating still has issue, and doesn't work on raven at current. So
disable it for the moment.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org