For virtual display, it uses software timer to emulate the vsync interrupt,
it doesn't have high precision, so doesn't support disable vblank immediately.

BUG: SWDEV-129274
Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>