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_crtc_internal.h b/drivers/gpu/drm/drm_crtc_internal.h
index 955c569..81bc0a3 100644
--- a/drivers/gpu/drm/drm_crtc_internal.h
+++ b/drivers/gpu/drm/drm_crtc_internal.h
@@ -142,6 +142,7 @@ int drm_mode_connector_set_obj_prop(struct drm_mode_object *obj,
 				    struct drm_property *property,
 				    uint64_t value);
 int drm_connector_create_standard_properties(struct drm_device *dev);
+const char *drm_get_connector_force_name(enum drm_connector_force force);
 
 /* IOCTL */
 int drm_mode_connector_property_set_ioctl(struct drm_device *dev,