intel: Convert to using private depth/stencil buffers (v2)

intel: Convert to using private depth/stencil buffers (v2)

This means that GLX buffer sharing of these no longer works.  On the
other hand, just *look* at this code reduction.

v2:
  - [chad] Fix intelCreateBuffer for gen < 6. When the branch for
    !screen->hw_has_separate_stencil was taken,
    intel_create_private_renderbuffer was incorrectly not used.

  - [chad] Remove all code in intel_process_dri2_buffer for processing
    depth, stencil, and hiz buffers. That code is now dead.

CC: Eric Anholt <eric@anholt.net>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>