drm/i915: Only signal from interrupt when requested

Avoid calling dma_fence_signal() from inside the interrupt if we haven't
enabled signaling on the request.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180627201304.15817-4-chris@chris-wilson.co.uk