On future platforms the index of the color-clear plane will change from
the one used by the GEN12 RC CCS CC modifier, so add a way to retrieve
the index independently of the platform/modifier.

Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020195138.1841242-8-imre.deak@intel.com