clk: mdss: add software mux for byte and pixel source clocks

Implement the software mux to byte clock and pixel source clock
with shadow implementation of PLL clock. This is used for
configuring the dynamic refresh pll registers.

Change-Id: I9c84cb76d040c5df7361291b6e1fc0fe69dc214f
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>