drm/i915/dsc: add support for computing and writing PPS for DSI encoders

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