drm/debugfs: Add internal client debugfs file

Print the names of the internal clients currently attached.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703160354.59955-6-noralf@tronnes.org