nir/spirv: Restrict the number of channels in texture coordinates

nir/spirv: Restrict the number of channels in texture coordinates

Some SPIR-V texturing instructions pack more than the texture coordinate
into the coordinate source.  We need to mask off the unused channels.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>