drm/amdgpu: disble vblank when unloading sriov driver

disble vblank in dce_vitual_crtc_commit(), which is skipped
under sriov before

Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Jiawei <Jiawei.Gu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>