i965: Add field intel_mipmap_tree::disable_aux_buffers

i965: Add field intel_mipmap_tree::disable_aux_buffers

The new field disables allocation of auxiliary buffers, such as the HiZ
buffer and MCS buffer. This is useful for sharing the miptree bo with an
external client that doesn't understand auxiliary buffers.

We need this field to safely render to a buffer that was imported with
EGL_EXT_image_dma_buf_import, because EGL does not yet have extensions
to manage flushing and invalidating auxiliary buffers.

Nothing yet enables this field. That's left to follow-up patches.

Testing:
  - Tested on Ivybridge Chromebook Pixel with WebGL Aquarium and
    YouTube.
  - No Piglit regressions on Broadwell with `piglit run -p gbm
    tests/quick.py`.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>