drm/msm: update writeback to request max mixer of given modes

Update writeback to request max number of mixers of all given
modes specified in WB config ioctl. To support dynamic
mode/resolution change, worst case resource should be allocated
to avoid out of resource during transition.

Change-Id: I90d9f1e8461dff25fd1d36385bc580958dbc736b
Signed-off-by: Alan Kwong <akwong@codeaurora.org>