UPSTREAM: 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


(cherry picked from commit 12f84e87)

BUG=b:308455498
TEST=Verify Asus PG32UQXR 4K160 mode on Rex using Big Joiner

Change-Id: I4aa25aae97b958ce79239c04fe121154a6e97d15
Signed-off-by: Rakshith M O <rakshith.m.o@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/5802871


Reviewed-by: Sean Paul <sean@poorly.run>
Reviewed-by: Manasi Navare <navaremanasi@chromium.org>
Signed-off-by: Hubert Mazur <hmazur@google.com>