drm/gud: Split up gud_flush_work()

In preparation for inlining synchronous flushing split out the part of
gud_flush_work() that can be shared by the sync and async code paths.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221122-gud-shadow-plane-v2-3-435037990a83@tronnes.org