drm/connector: Add helper to get a connector type name

drm_connector.c contains a map of connector types (DRM_MODE_CONNECTOR_*)
to name strings, but doesn't expose it. This leads to drivers having to
store a similar map.

Add a new drm_get_connector_type_name() helper function that return a
name string for a connector type.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-3-laurent.pinchart@ideasonboard.com