msm: mdss: update the dynamic refresh registers only for PLL0

Currently, when changing the refresh rate dynamically, the same
DSI PLL0 registers get updated as we dont use the PLL1. This
change avoids programmaing of second controller.

Change-Id: I599787f25fe3f197c426e38c0a02fa367ec2732e
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>