drm/i915: Update memory bandwidth parameters

Earlier while calculating derated bw we would use 90% of the calculated
bw. Starting ADL-P we use a non standard derating. Updating the formulae
to reflect the same.

Bspec: 64631

v2: Use the new derating value only for ADL-P(MattR)

Fixes: 4d32fe2f ("drm/i915/adl_p: Update memory bandwidth parameters")
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210914220744.16042-1-radhakrishna.sripada@intel.com