List of values like the DRM_MODE_SUBCONNECTOR_xx ones are better
represented with enums.

Turn the DRM_MODE_SUBCONNECTOR_xx macros into an enum.
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Suggested-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NEric Anholt <eric@anholt.net>