This avoids redundant per-layer operations that are the same across
layers or that only need to do once. Namely:

- The sampler for the blit source is the same for all layers.
- The decision about whether we need to load TLB contents or not only
  needs to be done once.
- Some command buffer state such as the pipeline, the viewport and the
  scissor is the same for all layers and should only be bound once.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <mesa/mesa!7651>