Switch off the scanout during driver unregister, so we can shutdown the
HW immediately for unbind.

v2: Remove the old shutdown from remove, it should now be redundant.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201204161601.20897-1-chris@chris-wilson.co.uk