Add DSI specific computation and transmission to display of PPS.

With hopes that this approach will work for both DP and DSI encoders.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/461db10b1f4d76625625a9f2b1e3d932fff42799.1575974743.git.jani.nikula@intel.com