imx-drm: imx-drm-core: fix DRM cleanup paths

We must call drm_vblank_cleanup() on the error cleanup and unload paths
after we've had a successful call to drm_vblank_init().  Ensure that
the calls are in the reverse order to the initialisation order.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>