msm: mdss: Reserve single SMP block for writeback mixer pipes

Writeback does not have stringent requirement for latency
buffers. So, SMP blocks reservation for the pipes attached
to writeback block can be restricted to single SMP block for
each plane. This makes more SMP block available for other
clients.

Change-Id: I3d788bcf7fcbc5667b4a3a0c52256f55cc0aa1e7
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>