msm_fb: Return pan_display call on Vsync interrupt

Till now, the pan_display call returns on DMA_P_DONE interrupt.
For command mode panels, the DMA transfer starts only after the
DMA engine receives a TE signal from the panel. Hence, return
pan_display ioctl on vsync interrupt to improve performance.
We still wait if the previous DMA transfer is not finished at
the beginning of next pan_display.

Change-Id: Ie92bc000a5842aa02a77a08b6bafdc8472bbebbd
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>