nv30, nv40: unify nv[34]0_miptree.c

nv30, nv40: unify nv[34]0_miptree.c

The only difference between nv30 and nv40 is that nv30 allowed swizzling
for more texture types.

This patch preserves the existing behavior, using conditional code.

Note however that this does not make sense, since all texture types can
be swizzled on nv40 and probably on nv30 too.

However, the handling of swizzled surfaces in the current 2D code is
partially broken, so it's best not to touch this.

A whole rewrite of the 2D code will be submitted, which will solve this
problem.