msm: mdss: add 8996 dsi pll support

Implement dsi related clocks framework so that dsi
vco pll related function be called to output correct
vco rate base on byte clock rate. After that pixel
clock rate can be achieved through MND setting.

Change-Id: I819f9fcb8afd9430f131679434c4da34641ce3f8
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>