msm: ndss: dsi: fix PLL source configuration for branch clocks

The PLL source for the DSI clocks can be specified using the
pll_src_config DTSI binding. However, this binding is optional and
whenever it is not specified, the default configuration should be
programmed based on the underlying hardware configuration. If the
hardware configuration is split-dsi, then both branches should source
from the same PLL, otherwise they should source from their corresponding
PLLs.

Change-Id: Idc1c003fede7440bad10311fe1f3bc44cc627053
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>