drm/i915: Enable irq to trace batch buffer completion.

If we trigger a tracepoint for batch buffer submission, it is a reasonable
assumption that we wish to also trace the batch buffer completion. So in
order to capture the completion events, we need to enable irqs... However,
we cannot rely on the completion event to disable the irq later, so we
defer the irq disable to the retire request.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>