msm_fb: display: extend mutex protection for mdp pan display

When mdp commits a buffer to play, functions inside pan display, e.g.
mdp4_overlay_mdp_perf_upd are based on the current state of play, so
extending mutex to the scope of pan display to avoid race conditions.

Change-Id: I2e55567fd21de3738be066a2dee298d8122f12a3
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>