intel: Replace intel_renderbuffer::region with a miptree [v3]

Essentially, this patch just globally substitutes `irb->region` with
`irb->mt->region` and then does some minor cleanups to avoid segfaults
and other problems.

This is in preparation for
  1. Fixing scatter/gather for mipmapped separate stencil textures.
  2. Supporting HiZ for mipmapped depth textures.

As a nice benefit, this lays down some preliminary groundwork for easily
texturing from any renderbuffer, even those of the window system.

A future commit will replace intel_mipmap_tree::hiz_region with a miptree.

v2:
   - Return early in intel_process_dri2_buffer_*() if region allocation
     fails.
   - Fix double semicolon.
   - Fix miptree reference leaks in the following functions:
       intel_process_dri2_buffer_with_separate_stencil()
       intel_image_target_renderbuffer_storage()

v3:
   - [anholt] Fix check for hiz allocation failure. Replace
     ``if (!irb->mt)` with ``if(!irb->mt->hiz_region)``.

Reviewed-by: Eric Anholt <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>
Signed-off-by: Chad Versace <[email protected]>

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