drm/i915: Split plane data_rate into data_rate+data_rate_y

Split the currently combined plane data_rate into the proper
Y vs. CbCr components. This matches how we now track the
plane dbuf allocations, and thus will make the dbuf bandwidth
calculations actually produce the correct numbers for each
dbuf slice.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220303191207.27931-3-ville.syrjala@linux.intel.com