drm/i915/dsc: clean up rc parameter table access

Use a simple pointer to the relevant element instead of duplicating the
array subscription. No functional changes.

Cc: Manasi Navare <manasi.d.navare@intel.com>
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/20191104141439.26312-2-jani.nikula@intel.com