clk: mdss: shadow clock implementation for dynamic refresh rate

This change implements shadow software mux and clocks for updating
dynamic refresh registers through 14nm DSI PLL calculations.

Change-Id: I461402a249d842d1fb2a8e9ecf82c2c87a37691c
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>