drm: drm_minor_register(): Clean up debugfs on failure

Call drm_debugfs_cleanup() in case drm_debugfs_init() fails to
cover for failure in the drm_driver.debugfs_init callback.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126225621.12314-3-noralf@tronnes.org