Initialize overlay->last_flip properly instead of leaving it zeroed.

Cc: stable@vger.kernel.org
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 0d9bdd88 ("drm/i915: Convert intel_overlay to request tracking")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161221144547.27319-1-ville.syrjala@linux.intel.com