drm/amdgpu/dce_virtual: Enable vBlank control for vf

This function actually control the vblank on/off.
It shouldn't be bypassed for VF. Otherwise all the
vblank based feature on VF will not work.

Signed-off-by: shaoyunl <shaoyun.liu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>