msm: mdss: change MDP writeback priority

Default priority for writeback is set to lowest, which can lead to other
clients' requests to come before MDP writeback. Change the priority to
P1 to match the rotator fetch QoS settings.

Change-Id: Ia703d1688581a0811502ce00863d1c973d53e749
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>