msm: mdss: add support for dedicated WB mixer

In some MDP revisions, the layermixer which is
attached to writeback block, is dedicated. It cannot
be used for interfaces. Adding support to take care
of mixer allocations for WB and rotator paths,
for such MDP hardware.

Change-Id: I9df51cf419c1dae3d2ce10597017a05dca1023a8
Signed-off-by: Krishna Chaitanya Devarakonda <kdevarak@codeaurora.org>