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 prepare_fence
callback to use before waiting for pending_crtcs_event.

Change-Id: Ifa2298cd030720cc0e7dcc0f80e63326b3e86768
Signed-off-by: Yujun Zhang <yujunzhang@codeaurora.org>