drm: Introduce a drm_crtc_commit_wait helper

There's currently four users of the same logic to wait for a commit to
be flipped: three for the CRTCs, connectors and planes in
drm_atomic_helper_wait_for_dependencies, and one in vc4.

Let's consolidate this a bit to avoid any code duplication.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111084401.117152-1-maxime@cerno.tech

include/drm/drm_atomic.h

@@ -66,6 +66,8 @@
  *
  * For an implementation of how to use this look at
  * drm_atomic_helper_setup_commit() from the atomic helper library.
+ *
+ * See also drm_crtc_commit_wait().
  */
 struct drm_crtc_commit {
 	/**
@@ -436,6 +438,8 @@ static inline void drm_crtc_commit_put(struct drm_crtc_commit *commit)
 	kref_put(&commit->ref, __drm_crtc_commit_free);
 }
 
+int drm_crtc_commit_wait(struct drm_crtc_commit *commit);
+
 struct drm_atomic_state * __must_check
 drm_atomic_state_alloc(struct drm_device *dev);
 void drm_atomic_state_clear(struct drm_atomic_state *state);