drm/i915/dsc: fix DSC register selection for ICL DSI transcoders

ICL eDP and DSI transcoders have a DSC engine separate from the
pipe. Abstract the register selection and fix it for ICL.

Add a warning for pipe A DSC on ICL; it does not exist.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@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/01bcddcdf397b1c8eb859ed18ebe023fb64383d9.1576081155.git.jani.nikula@intel.com