drm/i915/gt: Ensure that all new contexts clear STOP_RING

Set up the RING_MI_MODE in new contexts to clear the STOP_RING bit, just
in case they find it still set after a reset (as they are the first
contexts to be run).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191229183153.3719869-1-chris@chris-wilson.co.uk