drm/atomic: Make atomic helper track newly assigned planes correctly, v2.

Commit 669c9215af ("drm/atomic: Make async plane update checks work as
intended, v2.") forced planes to always be tracked, but forgot to
explicitly get the crtc commit from the new crtc when available.

This broke plane commit tracking, and caused kms_atomic_transitions
to randomly fail with -EBUSY.

Changes since v1:
- Prefer new_crtc_state->crtc above old_crtc_state->crtc.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: 669c9215af ("drm/atomic: Make async plane update checks work as intended, v2.")
Cc: Gustavo Padovan <gustavo.padovan@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102671
Testcase: kms_atomic_transitions
Link: https://patchwork.freedesktop.org/patch/msgid/20171017052047.8983-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_atomic_helper.c

@@ -1801,7 +1801,7 @@ int drm_atomic_helper_setup_commit(struct drm_atomic_state *state,
 		    !try_wait_for_completion(&old_plane_state->commit->flip_done))
 			return -EBUSY;
 
-		commit = crtc_or_fake_commit(state, old_plane_state->crtc);
+		commit = crtc_or_fake_commit(state, new_plane_state->crtc ?: old_plane_state->crtc);
 		if (!commit)
 			return -ENOMEM;