drm/fsl: don't use drm_put_dev

fsl is already fully demidlayered in the probe function, but for
convenience stuck with drm_put_dev. Call the unregister/unref parts
separately, to make sure this driver works correct.

v2: Rebase.

Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Stefan Agner <stefan@agner.ch>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213175449.24525-1-daniel.vetter@ffwll.ch