msm: mdss: Restrict SMP buffer reservation

Given two line latency buffer requirement for performance, in
some cases, three or more SMP blocks get reserved. This happens
typically for the usecases involving higher resolution layers
(e.g. 1080P). For these usecases required number of SMP blocks
exceed total number of SMP blocks available, thus causing SMP
reservation failures. These failures are often encountered in
8x26 target as it has in total, only seven SMP blocks available.

Restrict SMP buffer reservation to two SMP blocks, based on an
optionable device tree property, to be able to support most of
the usecases. This restriction is not applicable to horizontal
flip cases where two line latency buffer reservation is
necessary.

Change-Id: I7d57568b4b2710df0142bbc16749e56a9b492dfe
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>