drm/omap: Remove omap_debugfs_cleanup()

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

Cc: tomi.valkeinen@ti.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126225621.12314-12-noralf@tronnes.org