drm: Turn DRM_MODE_SUBCONNECTOR_xx definitions into an enum

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: Boris Brezillon <boris.brezillon@free-electrons.com>
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>