gallium/u_gen_mipmap: rewrite using pipe->blit (v2)

gallium/u_gen_mipmap: rewrite using pipe->blit (v2)

This replaces u_gen_mipmap with an extremely simple implementation based
on pipe->blit. st/mesa is also cleaned up.

Pros:
- less code
- correct mipmap generation for NPOT 3D textures (u_blitter uses a better
  formula)
- queries are not affected by mipmap generation if drivers disable them

v2: add "first_layer", "last_layer" parameters, drop "face"
v2.1: add format
v2.2: document the format parameter