drm: Remove struct drm_flip_task from DRM interfaces

Contain struct drm_flip_task and its helper functions
drm_flip_work_allocate_task() and drm_flip_work_queue_task() within
drm_flip_work.c There are no callers outside of the flip-work code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20231101103618.23806-2-tzimmermann@suse.de