disp: msm: sde: fix the race condition with wb output buffers

In the case of concurrent writeback, display thread
does not wait for writeback done and the fences are released
immediately if the pending frame count is one or zero.
Also during this skip wait process, the output frame buffers are also
cleaned up. If there are couple of bad writeback frames, there could
be a race condition between frame buffer cleanup in the display driver
and DRM_IOCTL_MODE_RMFB ioctl from userspace. Hence add appropriate
ref count logic with output frame buffers while the driver is using them.

Change-Id: I1cf919b93424011c75c39bcddd296a03a9d5c4ee
Signed-off-by: Lakshmi Narayana Kalavala <lkalaval@codeaurora.org>