msm: mdss: add support for configuring fb format

With current implementation frame buffer format is
fixed to RGBA8888. Add support to configure the same
based on target through dtsi.

Change-Id: I60de1d6c34f324c17ca83ed46743e29b9f17d71c
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>