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: Nshaoyunl <shaoyun.liu@amd.com>
Reviewed-by: NHawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>