msm: mdss: fix wb memory registration optimization

In order to avoid multiple mappings of same buffer handle, we check for
any existing handles that are already mapped within writeback. However
current checks are based on file descriptors, and these may be reused
for a new buffer handle, and in turn we end up having wrong assumptions
of the actual queued buffer while performing the writeback commit.

Fix this by getting the actual ion handle mapped to the file descriptor
and look for this handle instead.

Change-Id: Ib800dc4000ce8353ed16c7e779cda21644696ec8
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>