Revert "i965/fs: Correctly set up gl_FragCoord.w on Sandybridge."

This reverts commit 4a3b28113c3d23ba21bb8b8f5ebab7c567083a6d, as it
caused a regression on Ironlake (bug #34646).

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

@@ -495,7 +495,7 @@ fs_visitor::emit_fragcoord_interpolation(ir_variable *ir)
    wpos.reg_offset++;
 
    /* gl_FragCoord.w: Already set up in emit_interpolation */
-   emit(fs_inst(BRW_OPCODE_MOV, wpos, this->pixel_w));
+   emit(fs_inst(BRW_OPCODE_MOV, wpos, this->wpos_w));
 
    return reg;
 }