freedreno: drop unused fd_fence_ref param

freedreno: drop unused fd_fence_ref param

The pscreen param was just there to satisfy pipe_screen::fence_reference

But some of the internal uses passed NULL for screen.  Which is a bit
ugly.  Instead drop the param and add a shim function to plug into the
screen.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Eric Anholt <eric@anholt.net>