gallivm: adjust wrap mode to CLAMP_TO_EDGE always for cube maps.

gallivm: adjust wrap mode to CLAMP_TO_EDGE always for cube maps.

Technically without seamless filtering enabled GL allows any wrap mode, which
made sense when supporting true borders (can get seamless effect with border
and CLAMP_TO_BORDER), but gallium doesn't support borders and d3d9 requires
wrap modes to be ignored and it's a pain to fix up the sampler state (as it
makes it texture dependent). It is difficult to imagine a situation where an
app really wants another behavior so just cheat here. (It looks like some
graphics hw (intel) actually requires this too hence it should be safe.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>