i965/fs: Use a default Y coordinate of 0 for TXF on gen9+

Previously, we were incrementing length but not actually putting anything
in the Y coordinate.  This meant that 1-D TXF operations had a garbage
array index.  If the surface is emitted as 1-D non-array, the coordinate
gets discarded and it works fine.  If it happens to be bound as an array
surface, it may count as an out-of-bounds array access and you get zero.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: "11.1 11.2 12.0" <mesa-stable@lists.freedesktop.org>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -4264,6 +4264,8 @@ lower_sampler_logical_send_gen7(const fs_builder &bld, fs_inst *inst, opcode op,
          if (coord_components >= 2) {
             bld.MOV(retype(sources[length], BRW_REGISTER_TYPE_D),
                     offset(coordinate, bld, 1));
+         } else {
+            sources[length] = brw_imm_d(0);
          }
          length++;
       }