drm/exynos: call drm_put_dev directly from ->remove

I didn't find any user of the driver data yet, so store the
drm_device pointer in there.

Cc: Inki Dae <inki.dae@samsung.com>
Acked-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>