gallivm: Fix trivial out-of-bounds indirection in lp_build_cube_lookup().

gallivm: Fix trivial out-of-bounds indirection in lp_build_cube_lookup().

Courtesy of clang:

  src/gallium/auxiliary/gallivm/lp_bld_sample.c:1483:10: warning: array index of '2' indexes past the end of an array (that contains 2 elements) [-Warray-bounds]
           tmp[2] = lp_build_swizzle_aos(coord_bld, ddx_ddy[1], swizzle02);
           ^   ~
  src/gallium/auxiliary/gallivm/lp_bld_sample.c:1430:10: note: array 'tmp' declared here
           LLVMValueRef ddx_ddy[2], tmp[2], rho_vec;
           ^
  src/gallium/auxiliary/gallivm/lp_bld_sample.c:1487:56: warning: array index of '2' indexes past the end of an array (that contains 2 elements) [-Warray-bounds]
              rho_vec = lp_build_add(coord_bld, rho_vec, tmp[2]);
                                                       ^   ~
  src/gallium/auxiliary/gallivm/lp_bld_sample.c:1430:10: note: array 'tmp' declared here
           LLVMValueRef ddx_ddy[2], tmp[2], rho_vec;
           ^
  src/gallium/auxiliary/gallivm/lp_bld_sample.c:1491:56: warning: array index of '2' indexes past the end of an array (that contains 2 elements) [-Warray-bounds]
              rho_vec = lp_build_max(coord_bld, rho_vec, tmp[2]);
                                                       ^   ~
  src/gallium/auxiliary/gallivm/lp_bld_sample.c:1430:10: note: array 'tmp' declared here
           LLVMValueRef ddx_ddy[2], tmp[2], rho_vec;
           ^