include/drm: Reword debug categories comment.

The debug category comment mentions 4 categories, but
more than 4 categories are listed. Let's change the
wording to something a bit more generic.
Signed-off-by: Robert Foss <robert.foss@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460644456-9752-1-git-send-email-robert.foss@collabora.com

include/drm/drmP.h

@@ -90,7 +90,7 @@ struct reservation_object;
 struct dma_buf_attachment;
 
 /*
- * 4 debug categories are defined:
+ * The following categories are defined:
  *
  * CORE: Used in the generic drm code: drm_ioctl.c, drm_mm.c, drm_memory.c, ...
  *	 This is the category used by the DRM_DEBUG() macro.