drm/xe: Fix comment on Wa_22013088509

On i915 the "see comment about Wa_22013088509" referred to the comment
in the graphics version >= 11 branch, where there were more details
about it. From the platforms supported by xe, only PVC needs
Wa_22013088509, but as the comment says, it's simpler to do it for all
platforms as there is no downside.  Bring the missing comment over from
i915 and reword it to fit xe better.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20230504073250.1436293-3-lucas.demarchi@intel.comSigned-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_gt_mcr.c

@@ -461,9 +461,16 @@ static u32 rw_with_mcr_steering(struct xe_gt *gt, struct xe_reg_mcr reg_mcr,
 	}
 
 	/*
-	 * Always leave the hardware in multicast mode when doing reads
-	 * (see comment about Wa_22013088509 below) and only change it
-	 * to unicast mode when doing writes of a specific instance.
+	 * Always leave the hardware in multicast mode when doing reads and only
+	 * change it to unicast mode when doing writes of a specific instance.
+	 *
+	 * The setting of the multicast/unicast bit usually wouldn't matter for
+	 * read operations (which always return the value from a single register
+	 * instance regardless of how that bit is set), but some platforms may
+	 * have workarounds requiring us to remain in multicast mode for reads,
+	 * e.g. Wa_22013088509 on PVC.  There's no real downside to this, so
+	 * we'll just go ahead and do so on all platforms; we'll only clear the
+	 * multicast bit from the mask when explicitly doing a write operation.
 	 *
 	 * No need to save old steering reg value.
 	 */