st/nine: Use fast clears more often for MRTs

st/nine: Use fast clears more often for MRTs

This enables to use fast clears in the following
case:

pixel shader renders to 1 RT
4 RT bound
clear
new pixel shader bound that renders to 4 RTs

Previously the fast clear path wouldn't be hit,
because when trying the fast clear path,
the framebuffer state would be configured for 1 RT,
instead of 4.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>