drm/imx: Annotate dma-fence critical section in commit path

drm_atomic_helper_commit_hw_done() is the last thing (no plane cleanup
apparrently), so it's the entire function. And a nice comment
explaining why the wait_for_flip_done is ahead, unlike the usual
sequence.

Aside, I think since the atomic helpers do track plane disabling now
separately this might no longer be a real problem since:

commit 21a01abbe32a3cbeb903378a24e504bfd9fe0648
Author: Maarten Lankhorst <[e-mail]>
Date: Mon Sep 4 12:48:37 2017 +0200

drm/atomic: Fix freeing connector/plane state too early by tracking commits, v3.

Plus the subsequent bugfixes of course, this was tricky to get right.

Signed-off-by: Daniel Vetter <[e-mail]>
Cc: Philipp Zabel <[e-mail]>
Cc: Shawn Guo <[e-mail]>
Cc: Sascha Hauer <[e-mail]>
Cc: Pengutronix Kernel Team <[e-mail]>
Cc: Fabio Estevam <[e-mail]>
Cc: NXP Linux Team <[e-mail]>
Cc: [e-mail]
Signed-off-by: Philipp Zabel <[e-mail]>

drivers/gpu/drm/imx/imx-drm-core.c

···
 	struct drm_plane_state *old_plane_state, *new_plane_state;
 	bool plane_disabling = false;
 	int i;
+	bool fence_cookie = dma_fence_begin_signalling();
 
 	drm_atomic_helper_commit_modeset_disables(dev, state);
 
···
 	}
 
 	drm_atomic_helper_commit_hw_done(state);
+	dma_fence_end_signalling(fence_cookie);
 }
 
 static const struct drm_mode_config_helper_funcs imx_drm_mode_config_helpers = {