drm: Add drm_vblank_work_flush_all().

In some cases we want to flush all vblank work, right before vblank_off
for example. Add a simple function to make this possible.

Check that both pending_work and running work are empty when flushing.

Co-Developed-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240522053341.137592-2-maarten.lankhorst@linux.intel.com