drm: debugfs: Remove all files automatically on cleanup

Instead of having the drivers call drm_debugfs_remove_files() in
their drm_driver->debugfs_cleanup hook, do it automatically by
traversing minor->debugfs_list.
Also use debugfs_remove_recursive() so drivers who add their own
debugfs files don't have to keep track of them for removal.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126225621.12314-2-noralf@tronnes.org