zink: use ConstOffset for nir_tex_src_offset

Quote from the OpenGL Shading Language spec, version 4.40, section 8.9.2 "Texel Lookup Functions":

The offset value must be a constant expression.

So, until we start consuming SPIR-V shaders, it seems we don't need to deal with non-constant offsets.

This means we can avoid lowering this away in some cases.

Closes: #4039 (closed)