drm/imx: fix commit_tail for new drm_atomic_helper_setup_commit

Since commit 080de2e5be2d ("drm/atomic: Check for busy planes/connectors before
setting the commit"), drm_atomic_helper_setup_commit expects that blocking
commits have completed flipping before the commit_tail returns. Add the missing
wait_for_flip_done to commit_tail to ensure this.
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