msm: mdss: add support to configure DSI branch clock source

Based on the board configuration, the two DSI controllers can either
drive two independent panels or drive a single panel in split display
mode. DSI0 PLL can drive split display configuration ands it is not
supported by DSI1 PLL. DSI0 PLL can drive a single DSI panel on either
DSI0 or DSI1 controller whereas DSI1 PLL can drive a DSI panel only on
DSI1 controller. Based on this, it is necessary to configure the
source for the DSI link clocks dynamically. Implement this by adding
support for a new pll source configuration entry.

Change-Id: Ie57afbfe8cb14b80d0b52ed3825972ae34af27e2
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>