msm_fb: display: fix mdp underrun on disabling writeback

Writeback fallback for mdp underflow can be enabled/disabled dynamically
based on parameters; if writeback is disabled while displaying previous
frame it can can cause underrun. Disable writeback after programming new
frame to avoid underrun.

Change-Id: I6c73887043f19596cb3611c83c51c1aeac907ff0
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>