drm/i915/opregion: debug log about Mailbox #2 for backlight

Start debug logging about the presence of the new Mailbox #2 for
backlight. Actual support is to be added later.

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/42fd9cd777c5cc9a8d48db9dd8306924c735918e.1644489329.git.jani.nikula@intel.com

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

@@ -47,10 +47,11 @@
 #define OPREGION_ASLE_EXT_OFFSET	0x1C00
 
 #define OPREGION_SIGNATURE "IntelGraphicsMem"
-#define MBOX_ACPI      (1<<0)
-#define MBOX_SWSCI     (1<<1)
-#define MBOX_ASLE      (1<<2)
-#define MBOX_ASLE_EXT  (1<<4)
+#define MBOX_ACPI		BIT(0)	/* Mailbox #1 */
+#define MBOX_SWSCI		BIT(1)	/* Mailbox #2 (obsolete from v2.x) */
+#define MBOX_ASLE		BIT(2)	/* Mailbox #3 */
+#define MBOX_ASLE_EXT		BIT(4)	/* Mailbox #5 */
+#define MBOX_BACKLIGHT		BIT(5)	/* Mailbox #2 (valid from v3.x) */
 
 struct opregion_header {
 	u8 signature[16];
@@ -957,6 +958,10 @@ int intel_opregion_setup(struct drm_i915_private *dev_priv)
 		opregion->asle_ext = base + OPREGION_ASLE_EXT_OFFSET;
 	}
 
+	if (mboxes & MBOX_BACKLIGHT) {
+		drm_dbg(&dev_priv->drm, "Mailbox #2 for backlight present\n");
+	}
+
 	if (intel_load_vbt_firmware(dev_priv) == 0)
 		goto out;