drm/i915: Do not check or a stalled pageflip prior to it being queued

When we queue the command or operation to change the scanout address, we
mark the flip as in progress. We can use this flag to prevent us from
checking for a stalled flip prior to its existence!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>