Without this, since the conversion from drm_vblank_pre/post_modeset to
drm_vblank_on/off, the vblank interrupt could never be disabled after
userspace triggered enabling it.
Signed-off-by: NMichel Dänzer <michel.daenzer@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>