nv50,nvc0: enable/disable seamless cubemap texturing as requested

nv50,nvc0: enable/disable seamless cubemap texturing as requested

In a situation where the seamless setting isn't available on a
per-texture basis (G200+ Teslas, and all Fermis), assume that all
samplers will have it identically set, and enable accordingly.

This fixes arb_seamless_cubemap piglit test on Fermi and Tesla.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>