msm_fb: writeback panel commit should not happen when buffer is NULL

The change handles the NULL buffer case for writeback panel.
It avoids the H/W commit when buffer is NULL and also adds it to
free queue.

CRs-Fixed: 448340
Change-Id: I60f5d343d6de0f96353b081aa113f347c73c61af
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>
msm: display: reset vsync state when display is turned on

In the case that vsync ctrl is unbalanced, vsync IRQ is cleared
in suspend,  vsync cannot be enabled after resume.
Clear the status when display is turned on

CRs-fixed: 449155
Change-Id: I5ecf23053b60d1fbd91f042758d67d39c83ab444
Signed-off-by: Ken Zhang <kenz@codeaurora.org>