msm: msm_fb: wait for VSYNC instead of DMA_P for commit completion.

In the current implementation, we are waiting for DMA_P interrupt
in commit call to signal completion. This can cause 2 commits happening
in the same vsync period due to which IOMMU page faults can happen when
vsync comes while second commit is programming resgisters. To avoid this,
wait for DMA_P is replaced by wait for VSYNC.

Change-Id: I68d4d4b4e7a574f86faecd5b4812e1f5852f4209
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>