drm: add drm_get_connector_force_name

Follow the naming in debugfs also for logging, add "unknown" for values
beyond the enumerated ones.

v2: add \n in connector_show, make internal to drm (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487580708-29340-1-git-send-email-jani.nikula@intel.com
diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
index 2290a74..8b2c61a 100644
--- a/drivers/gpu/drm/drm_debugfs.c
+++ b/drivers/gpu/drm/drm_debugfs.c
@@ -261,30 +261,8 @@ int drm_debugfs_cleanup(struct drm_minor *minor)
 static int connector_show(struct seq_file *m, void *data)
 {
 	struct drm_connector *connector = m->private;
-	const char *status;
 
-	switch (connector->force) {
-	case DRM_FORCE_ON:
-		status = "on\n";
-		break;
-
-	case DRM_FORCE_ON_DIGITAL:
-		status = "digital\n";
-		break;
-
-	case DRM_FORCE_OFF:
-		status = "off\n";
-		break;
-
-	case DRM_FORCE_UNSPECIFIED:
-		status = "unspecified\n";
-		break;
-
-	default:
-		return 0;
-	}
-
-	seq_puts(m, status);
+	seq_printf(m, "%s\n", drm_get_connector_force_name(connector->force));
 
 	return 0;
 }