nv50/ir: ignore bias for samplerCubeShadow on nv50

Unfortunately there's no good way to do this on the nv50 shader isa.
Dropping the bias seems preferable to doing the compare post-filtering.

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