mach64: optimize wait_for_fifo

This is a simple optimization for fifo waiting that improves scrolling
performance by 5%. If the queue has more free entries that what we
consume, we can skip the costly register read next time.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Ville Syrjälä <syrjala@sci.fi>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>