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

drivers/gpu/drm/i915/display/intel_bw.c

@@ -379,7 +379,7 @@ static const struct intel_sa_info mtl_sa_info = {
 	.deburst = 32,
 	.deprogbwlimit = 38, /* GB/s */
 	.displayrtids = 256,
-	.derating = 20,
+	.derating = 10,
 };
 
 static int icl_get_bw_info(struct drm_i915_private *dev_priv, const struct intel_sa_info *sa)