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 21a01abb
Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
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 <daniel.vetter@intel.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>