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

Cc: liviu.dudau@arm.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126225621.12314-9-noralf@tronnes.org