drm/i915/rkl: Update memory bandwidth parameters

The RKL platform has different memory characteristics from past
platforms.  Update the values used by our memory bandwidth calculations
accordingly.

Bspec: 53998
Cc: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200504225227.464666-7-matthew.d.roper@intel.com