drm/rockchip: shutdown drm subsystem on shutdown

As explained by Robin Murphy:
> the IOMMU shutdown disables paging, so if the VOP is still
> scanning out then that will result in whatever IOVAs it was using now going
> straight out onto the bus as physical addresses.

We had a more radical approach before in commit
7f3ef5de ("drm/rockchip: Allow driver to be shutdown on reboot/kexec")
but that resulted in new warnings and oopses on shutdown on rk3399
chromeos devices.

So second try is resurrecting Vicentes shutdown change which should
achieve the same result but in a less drastic way.

Fixes: 63238173

 ("Revert "drm/rockchip: Allow driver to be shutdown on reboot/kexec"")
Cc: Jeffy Chen <jeffy.chen@rock-chips.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Brian Norris <briannorris@chromium.org>
Cc: Doug Anderson <dianders@chromium.org>
Cc: stable@vger.kernel.org
Suggested-by: JeffyChen <jeffy.chen@rock-chips.com>
Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Vicente Bergas <vicencb@gmail.com>
[adapted commit message to explain the history]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Brian Norris <briannorris@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190402113753.10118-1-heiko@sntech.de