msm_fb: display: issue completion when disabling vsync irq

When vsync irq is disabled, vsync_wait_cnt is reset to zero and
completion is issued out as they are done in isr to make sure all
wait_for_completion is released.

Change-Id: I689f0c8cde43404d0caf5bb4644efa6385798d0e
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>