i965/fs: Get rid of fs_visitor::do_dual_src.

i965/fs: Get rid of fs_visitor::do_dual_src.

This boolean flag was being used for two different things:

 - To set the brw_wm_prog_data::dual_src_blend flag.  Instead we can
   just set it based on whether the dual_src_output register is valid,
   which will be the case if the shader writes the secondary blending
   color.

 - To decide whether to call emit_single_fb_write() once, or in a loop
   that would iterate only once, which seems pretty useless.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>