msm_fb: display: add mutex during lcdc off

There has possibility that suspend operation and screen
update happen simultaneously. There mutex at lcdc_off is
necessary to serialize both lcdc off and pan display.

Change-Id: I64c61d6be62ce518754571c3a364d27948b74a94
CRs-fixed: 302527
Signed-off-by: Khsieh <khsieh@codeaurora.org>