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

pixel_w is the final result; wpos_w is used on gen4 to compute it.

NOTE: This is a candidate for the 7.10 branch.

Reviewed-by: Eric Anholt <eric@anholt.net>

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

@@ -494,7 +494,7 @@ 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->wpos_w));
+   emit(fs_inst(BRW_OPCODE_MOV, wpos, this->pixel_w));
 
    return reg;
 }