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

Like the helpers, nothing special. Well except not, because we the
critical section extends until after hw_done(), since that's the last
thing which could hold up a subsequent atomic commit. That means the
wait_for_flip_done is included, but that's not a problem, we're
allowed to call dma_fence_wait() from signalling critical sections.
Even on our own fence (which this does), it's just a bit confusing.
But in a way those last 2 function calls are already part of the fence
signalling critical section for the next atomic commit.

Reading this I'm wondering why komeda waits for flip_done() before
calling hw_done(), which is a bit backwards (but hey hw can be
special). Might be good to throw a comment in there that explains why,
because the original commit that added this just doesn't.

v2: Small rebase

Reviewed-by: James Qian Wang <james.qian.wang@arm.com> (v1)
Cc: "James (Qian) Wang" <james.qian.wang@arm.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210121152959.1725404-5-daniel.vetter@ffwll.ch

drivers/gpu/drm/arm/display/komeda/komeda_kms.c

@@ -73,6 +73,7 @@ static const struct drm_driver komeda_kms_driver = {
 static void komeda_kms_commit_tail(struct drm_atomic_state *old_state)
 {
 	struct drm_device *dev = old_state->dev;
+	bool fence_cookie = dma_fence_begin_signalling();
 
 	drm_atomic_helper_commit_modeset_disables(dev, old_state);
 
@@ -85,6 +86,8 @@ static void komeda_kms_commit_tail(struct drm_atomic_state *old_state)
 
 	drm_atomic_helper_wait_for_flip_done(dev, old_state);
 
+	dma_fence_end_signalling(fence_cookie);
+
 	drm_atomic_helper_cleanup_planes(dev, old_state);
 }