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>

include/drm/drmP.h

@@ -1008,9 +1008,12 @@ struct drm_driver {
 	struct list_head legacy_dev_list;
 };
 
-#define DRM_MINOR_LEGACY 1
-#define DRM_MINOR_CONTROL 2
-#define DRM_MINOR_RENDER 3
+enum drm_minor_type {
+	DRM_MINOR_LEGACY,
+	DRM_MINOR_CONTROL,
+	DRM_MINOR_RENDER,
+	DRM_MINOR_CNT,
+};
 
 /**
  * Info file list entry. This structure represents a debugfs or proc file to