st/mesa: optionally apply texture swizzle to border color v2

st/mesa: optionally apply texture swizzle to border color v2

This is the only sane solution for nv50 and nvc0 (really, trust me),
but since on other hardware the border colour is tightly coupled with
texture state they'd have to undo the swizzle, so I've added a cap.

The dependency of update_sampler on the texture updates was
introduced to avoid doing the apply_depthmode to the swizzle twice.

v2: Moved swizzling helper to u_format.c, extended the CAP to
provide more accurate information.