drm/gm12u320: Simplify upload work

Instead of having a work item that never stops (which really should be
a kthread), with a dedicated workqueue to not upset anyone else, use a
delayed work. A bunch of changes:

- We can throw out all the custom wakeup and requeue logic and state
  tracking. If we schedule the work with a 0 delay it'll get
  scheduled immediately.

- Persistent state (frame & draw_status_timeout) need to be moved out
  of the work.

- diff is bigger than the changes, biggest chunk is reindenting the
  work fn because it lost its while loop.

Lots of code deleting as consequence all over. Specifically we can
delete the drm_driver.release code now!

v2: Review from Hans:
- Use mod_delayed_work in the plane update path to make sure we do
  actually schedule immediately). In the worker we still want
  queue_delayed_work, which won't modify the timeout when the work is
  already scheduled. Which is exactly what we want if the work races
  with a plane update.
- Switch to system_long_wq, Hans says on usb2 a plane upload can take
  80 ms.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-46-daniel.vetter@ffwll.ch