anv: Only enable EWA LOD algorithm when doing anisotropic filtering.

Updated documentation renames "Anisotropic Algorithm" to "LOD Algorithm"
and adds a note for Gen9+ saying "The EWA Algorithm should only be
enabled for Anisotropic Filtering modes." and indicating that the extra
accuracy shouldn't be necessary for other modes, and comes at a cost.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>