intel: Replace intel_mipmap_tree::hiz_region with a miptree [v2]

intel: Replace intel_mipmap_tree::hiz_region with a miptree [v2]

This is required to correctly implement HiZ for mipmapped and
multi-layered textures.

v2: Accomodate refcount fixes in intel_process_dri2_buffer_*() that were
    introduced in v2 of commit
        intel: Replace intel_renderbuffer::region with a miptree [v2]

Reviewed-by: Eric Anholt <eric@anholt>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>