msm_fb: display: replace msleep with wait4vsync

vsync is controlled by frame work which means vsync irq
off may happen at any instance. This patch use a reference
count to control vsync irq enable/disable so that
wait4vsync can replace msleep safely during suspend.

Change-Id: Ief84b1b421436e1f750288eb9a9760a9cb96c555
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>