drm/etnaviv: Remove etnaviv_debugfs_cleanup()

drm_debugfs_cleanup() now removes all minor->debugfs_list entries
automatically, so the drm_driver.debugfs_cleanup callback is not
needed.

Cc: l.stach@pengutronix.de
Cc: linux+etnaviv@armlinux.org.uk
Cc: christian.gmeiner@gmail.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126225621.12314-8-noralf@tronnes.org