drm/i915/execlists: Clear STOP_RING bit on reset

During reset, we try to ensure no forward progress of the CS prior to
the reset by setting the STOP_RING bit in RING_MI_MODE. Since gen9, this
register is context saved and do we end up in the odd situation where we
save the STOP_RING bit and so try to stop the engine again immediately
upon resume. This is quite unexpected and causes us to complain about an
early CS completion event!

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111514


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190910080208.4223-1-chris@chris-wilson.co.uk