i965/fs: Assert that the number of color targets is one when dual-source blend is enabled.

Requested by Anuj during review of
4a87e4ade7, adding as follow-up since it
led to assertion failures due to various GLSL bugs that should be
fixed now.

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

@@ -487,6 +487,7 @@ fs_visitor::emit_fb_writes()
    }
 
    prog_data->dual_src_blend = (this->dual_src_output.file != BAD_FILE);
+   assert(!prog_data->dual_src_blend || key->nr_color_regions == 1);
 
    if (inst == NULL) {
       /* Even if there's no color buffers enabled, we still need to send