drm/display/dsc: use flat array for rc_parameters lookup

Next commits are going to add support for additional RC parameter lookup
tables. These tables are going to use different bpp/bpc combinations,
thus it makes little sense to keep the 2d array for RC parameters.
Switch to using the flat array.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230517102807.2181589-6-dmitry.baryshkov@linaro.org
Acked-by: Dave Airlie <airlied@redhat.com>