msm_fb: Return ioctl on vsync interrupt disable

Before the vsync interrupt gets disabled, initialize the completion
structure and then disable the interrupt. This enables the ioctl to
return successfully in all the cases.

Change-Id: I6a7c233f818a773d00497502213d1ecf8fe9dc24
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>