drm/i915: add a note about fec_enable with 128b/132b

Add a note that fec_enable actually means FEC is to be enabled
explicitly. 128b/132b always has FEC enabled, the driver doesn't need to
enable it separately, and fec_enable will be false.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230920112901.3315876-1-jani.nikula@intel.com

drivers/gpu/drm/i915/display/intel_display_types.h

@@ -1365,7 +1365,12 @@ struct intel_crtc_state {
 
 	bool enhanced_framing;
 
-	/* Forward Error correction State */
+	/*
+	 * Forward Error Correction.
+	 *
+	 * Note: This will be false for 128b/132b, which will always have FEC
+	 * enabled automatically.
+	 */
 	bool fec_enable;
 
 	bool sdp_split_enable;