drm/i915/gt: Lift stop_ring() to reset_prepare

Push the sleeping stop_ring() out of the reset resume function to reset
prepare; we are not allowed to sleep in the former.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210119110802.22228-3-chris@chris-wilson.co.uk