drm/amdgpu/virtual_dce: Virtual display doesn't support disable vblank immediately

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>