drm/i915/gt: Disable the ring before resetting HEAD/TAIL

During the reset of ring submission, we first stop the engine by
clearing the HEAD/TAIL and marking the ring as disabled. However, it
would be safer to disable the ring (after emptying) before resetting the
HEAD/TAIL.

Suggested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
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/20210119110802.22228-4-chris@chris-wilson.co.uk