drm/amdgpu: fix to disable powergating in hw_fini

We need enable CGPG and GFXOFF together. If only enable one of them, this system
will get hang after startx (do draw command). So when gfxoff is disabled, it
also need disable CGPG after that.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>