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
such deadlock, make the modeset lock within prepare_fence
callback to use before waiting for pending_crtcs_event.

Change-Id: Iba05efae1bfab4f454083b6e462a59937d06a7e6
Signed-off-by: Mahadevan <mahap@codeaurora.org>