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