nv30: hack to avoid errors on unexpected color/zeta combinations

This is just a hack, it should be possible to create a temporary zeta
surface and render to that instead. However that's more complicated and
this avoids the render being entirely broken and errors being reported
by the card.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>
nv30: also unreference bufctx on context destroy

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>