i965/fs: Move sampler unit lookup into rescale_texcoord

i965/fs: Move sampler unit lookup into rescale_texcoord

The texunit variable we create and assign in nir_emit_texture gets passed
through two more layers of function calls before it gets to its sole use in
rescale_texcoord.  The best part is that we already pass the sampler into
rescale_texcoord so we can just look it up there.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>