msm: mdss: select mixer respective to ctl path for wb

MDSS driver reserves initial ctl path for primary display
for reduced wb interfaces. It uses rest ctl path for
other displays. Follow same allocation for mixer
path also.

Change-Id: I1bb079ae9a916a0addcb7ca8b16e4853f1bcab6a
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>