drm/amdgpu: should stop GFX ring in hw_fini

To align with the scheme from gfx9

disabling GFX ring after VM shutdown could avoid
garbage data be fetched to GFX RB which may lead
to unnecessary screw up on GFX

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>