u_simple_shaders: fix mask handling in util_make_fragment_tex_shader_writemask

u_simple_shaders: fix mask handling in util_make_fragment_tex_shader_writemask

The writemask handling was busted, since writing defaults to output
meant they got overwritten by the tex sampling anyway. Albeit the
affected components were undefined, so maybe with some luck it
still would have worked with some drivers - if not could as well
kill it... (This would have affected u_blitter but not u_blit since
the latter always used xyzw mask.)

Reviewed-by: Brian Paul <brianp@vmware.com>