drm/i915: Kill obj->pending_flip

The pending flip mask no longer set anywhere, so trying to wait for
while it's non-zero is a no-op. Remove it completely.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>