msm: mdss: use half of panel width when using dual pipe

MDP layer mixer only supports up to 2048 width, if the panel resolution
is greater than this dual mixer needs to be used. Setup each mixer width
as half the panel width instead of allocating max width on left mixer
and use left over for right mixer to have better load balancing.

Change-Id: Ibc01591e2e6d5e5f98e81dfba766493abd9895a5
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>