msm: mdss: Support the option to swap mixer muxes.

Different panel vendors have different board configurations.
Driver needs to support swapping mixer muxes to accomodate
all configurations if the swap property is defined in the
device tree.

CRs-fixed: 470035
Change-Id: I1c4299021a53c16ab2d5491b755c321f5e921c72
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>