ARM: dts: msm: Update MDSS mixers in msm8937 DT

msm8937 doesn't have writeback mixers instead has
two interface mixers. Update the same in MDSS DT.

Change-Id: I6db73e2b919526a1f10cab86f1bbc18ecff95ce5
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>