lavapipe: be slightly more permissive for bad apps (and cts) with dynrender

this is stupid, but it makes the default color output "write the output"
instead of zero, which is still wrong, but at least maybe it's wrong
in a way that's useful instead of just being confusing

Acked-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24278>

src/gallium/frontends/lavapipe/lvp_execute.c

@@ -907,6 +907,7 @@ static void handle_graphics_pipeline(struct lvp_pipeline *pipeline,
       }
    } else if (ps->rp->color_attachment_count == 0) {
       memset(&state->blend_state, 0, sizeof(state->blend_state));
+      state->blend_state.rt[0].colormask = 0xf;
       state->blend_dirty = true;
    }