drm/sun4i: Implement drm_driver lastclose to restore fbdev console

The drm_driver lastclose callback is called when the last userspace
DRM client has closed. Call drm_fbdev_cma_restore_mode to restore
the fbdev console otherwise the fbdev console will stop working.

Fixes: 9026e0d1 ("drm: Add Allwinner A10 Display Engine support")
Cc: stable@vger.kernel.org
Tested-by: Olliver Schinagl <oliver@schinagl.nl>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>