drm/msm/sde: allow writeback to support null fb

Allow writeback to support null fb so that user mode
can perform null commit to select drm mode for
topology determination.

Change-Id: I505f26da2555531a63e93a99cdfe49e5c01bce90
Signed-off-by: Alan Kwong <akwong@codeaurora.org>
6 files changed