drm/sun4i: Set device driver data at bind time for use in unbind

Our sun4i_drv_unbind gets the drm device using dev_get_drvdata.
However, that driver data is never set in sun4i_drv_bind.

Set it there to avoid getting a NULL pointer at unbind time.

Fixes: 9026e0d1

 ("drm: Add Allwinner A10 Display Engine support")
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190418132727.5128-3-paul.kocialkowski@bootlin.com