gallivm: fix texel fetch for array textures

gallivm: fix texel fetch for array textures

Since we don't call lp_build_sample_common() in the texel fetch path we missed
the layer fixup code. If someone would have tried to do texelFetch with array
textures it would have crashed for sure.
Not really tested (can't run the piglit test being able to use texelFetch with
array samplers for now with llvmpipe).

Reviewed-by: José Fonseca <jfonseca@vmware.com>