xf86drmMode: introduce drmModeGetConnectorTypeName

User-space often needs to print the name of a connector type.
When a new connector type is added, all user-space programs need
to be updated to support the new connector type.

Expose a function to get a connector type name in libdrm.

The names are taken from the kernel [1].

[1]: https://cgit.freedesktop.org/drm/drm/tree/drivers/gpu/drm/drm_connector.c?h=4fc8cb47fcfdc93e274a1291757e478df4f9c39b#n83



Signed-off-by: Simon Ser <contact@emersion.fr>