anv/wsi_wayland: Fix FIFO mode

anv/wsi_wayland: Fix FIFO mode

Previously, there were a number of things we were doing wrong:

 1) We weren't flushing the wl_display so dead-looping clients weren't
    guaranteed to work.
 2) We were sending the frame event after calling wl_surface.commit() so it
    wasn't getting assigned to the correct frame
 3) We weren't actually setting fifo_ready to false.

Unfortunately, we never noticed because (3) was hiding the other two.  This
commit fixes all three and clients that use FIFO mode are now properly
refresh-rate limited.