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>