msm: mdss: reduce timeline for writeback display

For writeback display, buffers can be released and signaled as soon as
commit is done since the contents will be pushed directly to memory.
That is, there is no need to hold buffer for extra timeline. Reduce
timeline by one for buffers pushed to writeback display to allow buffers
to be reused as soon as possible.

CRs-Fixed: 474903
Change-Id: I896803a12e5dde2983cb284d6914f195ac996337
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>