drm: turn DRM_MINOR_* into enum

Use enum for DRM_MINOR_* constants to avoid hard-coding the IDs.
Furthermore, add a DRM_MINOR_CNT so we can perform range-checks in
follow-ups.

This changes the IDs of the minor-types by -1, but they're not used as
indices so this is fine.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>