drm/i915: Wrap -EIO send-vblank event for failed pageflip in spinlock

drm_send_vblank_event() demands that we hold the event spinlock whilst
calling it, so do so.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Fix the double lock as requested by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>