Use the correct DSS CTL registers for ICL DSI transcoders.

As a side effect, this also brings back the sanity check for trying to
use pipe DSC registers on pipe A on ICL.

Fixes: 8a029c11 ("drm/i915/dp: Modify VDSC helpers to configure DSC for Bigjoiner slave")
References: http://lore.kernel.org/r/87eegxq2lq.fsf@intel.com
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: <stable@vger.kernel.org> # v5.11+
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210319115333.8330-1-jani.nikula@intel.com