amd/llvm: Fix warning due to asserted-only variable.

amd/llvm: Fix warning due to asserted-only variable.

[212/893] Compiling C object 'src/amd/llvm/ce8261c@@amd_common_llvm@sta/ac_nir_to_llvm.c.o'.
../mesa/src/amd/llvm/ac_nir_to_llvm.c: In function ‘visit_image_atomic’:
../mesa/src/amd/llvm/ac_nir_to_llvm.c:2636:17: warning: unused variable ‘format’ [-Wunused-variable]
 2636 |    const GLenum format = nir_intrinsic_format(instr);
      |                 ^~~~~~

Reviewed-by: Marek Olšák <marek.olsak@amd.com>