drm/uapi: Add USB connector type

Add a connector type for USB connected display panels.

Some examples of what current userspace will name the connector:
- Weston: "UNNAMED-%d"
- Mutter: "Unknown20-%d"
- X: "Unknown20-%d"

v2:
- Update drm_connector_enum_list
- Add examples to commit message

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210313112545.37527-2-noralf@tronnes.org