disp: msm: avoid deadlock by prepending connection_mutex

Commit caused by dpms on may use extra  modeset lock on
connection_mutex before msm_atomic_commit, so to avoid
deadlock, make the modeset lock within msm_atomic_commit
to be used before the waiting for pending_crtcs_event.

Change-Id: Ic43af0f775d87ccc1d145ead9cb2e1b65018c86f
Signed-off-by: Mahadevan <mahap@codeaurora.org>