Silently cancelling vblank works is a bit rude, especially
if said works do any resource management (eg. free memory).
WARN if we ever hit this.

TODO: Maybe drm_crtc_vblank_off() should wait for any
pending work to reach its target vblank before actually
doing anything drastic?

Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230920140339.28322-2-ville.syrjala@linux.intel.comReviewed-by: Lyude Paul <lyude@redhat.com>