drm/i915: Skip engine serialisation for no-op seqno reset

If the engine's seqno is already at our target seqno (most likely it
hasn't been used since the last reset), we can skip serialising the
engine and leave it as is.

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/20181126095610.20962-1-chris@chris-wilson.co.uk