draw: make sure that the stages setup outputs

draw: make sure that the stages setup outputs

Calling the prepare outputs cleans up the slot assignments
for outputs, unfortunately aapoint and aaline didn't have
code to reset their slots after the initial setup, this
was messing up our slot assignments. The unfilled stage
was just missing the initial assignment of the face slot.
This fixes all of the reported piglit failures.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>