drm/msm/mdp5: Fix wait-for-commit for cmd panels

Before the offending commit in msm_atomic_commit_tail wait_flush was
called once per frame, after the commit was submitted. After it
wait_flush is also called at the beginning to ensure previous
potentially async commits are done.

For cmd panels the source of wait_flush is a ping-pong irq notifying
a completion. The completion needs to be notified with complete_all so
multiple waiting parties (new async committers) can proceed.

Signed-off-by: Iskren Chernev <iskren.chernev@gmail.com>
Suggested-by: Rob Clark <robdclark@gmail.com>
Fixes: 2d99ced7 ("drm/msm: async commit support")
Signed-off-by: Rob Clark <robdclark@chromium.org>