drm/debug: Expose connector's max supported bpc via debugfs

It's useful to know the connector's max supported bpc for IGT
testing. Expose it via a debugfs file on the connector "output_bpc".

Example: cat /sys/kernel/debug/dri/0/DP-1/output_bpc

V2:
* Fix typo in comments (Harry)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220519095149.3560034-2-bhanuprakash.modem@intel.com

drivers/gpu/drm/drm_debugfs.c

@@ -395,6 +395,23 @@ static int vrr_range_show(struct seq_file *m, void *data)
 }
 DEFINE_SHOW_ATTRIBUTE(vrr_range);
 
+/*
+ * Returns Connector's max supported bpc through debugfs file.
+ * Example usage: cat /sys/kernel/debug/dri/0/DP-1/output_bpc
+ */
+static int output_bpc_show(struct seq_file *m, void *data)
+{
+	struct drm_connector *connector = m->private;
+
+	if (connector->status != connector_status_connected)
+		return -ENODEV;
+
+	seq_printf(m, "Maximum: %u\n", connector->display_info.bpc);
+
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(output_bpc);
+
 static const struct file_operations drm_edid_fops = {
 	.owner = THIS_MODULE,
 	.open = edid_open,
@@ -437,6 +454,10 @@ void drm_debugfs_connector_add(struct drm_connector *connector)
 	debugfs_create_file("vrr_range", S_IRUGO, root, connector,
 			    &vrr_range_fops);
 
+	/* max bpc */
+	debugfs_create_file("output_bpc", 0444, root, connector,
+			    &output_bpc_fops);
+
 	if (connector->funcs->debugfs_init)
 		connector->funcs->debugfs_init(connector, root);
 }