softpipe: fix using optimized filter function

softpipe: fix using optimized filter function

This optimized filter (when using repeat wrap modes,
linear min/mag/mip filters, pot textures) only applies to 2d textures,
but nothing prevented it from being used for other textures (likely
leading to very bogus sample results).

Note: This is a candidate for the 9.0 branch.

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