drm/i915: fix the derating percentage for MTL

Follow the values from bspec for the percentage overhead for
efficiency in MTL BW calculations.

Bspec: 64631

Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230606093509.221709-2-vinod.govindapillai@intel.com