nir: set default lod to texture opcodes that needed it but don't provide it

nir: set default lod to texture opcodes that needed it but don't provide it

v2:
- Use helper to add a new source to the texture instruction.

v3:
- Use nir_tex_instr_src_index() to simplify the patch (Jason).

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>