msm_fb: Use an exclusive semaphore for Vsync IOCTL

Vsync IOCTL runs in a separate thread context and is independent of the
other display IOCTLs. Right now, we have the same semaphore shared
for Vsync IOCTL along with other IOCTLs. This will cause performance
drop if a Vsync enable/disable IOCTL thread waits to acquire the
semaphore since composition might be happening through Overlay/Blit
IOCTL. Hence, use a different semaphore for the Vsync IOCTL.

Change-Id: I190505740b289b01812b2a46d237854a31d047e8
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>