intel: Add hiz_region to intel_renderbuffer

intel: Add hiz_region to intel_renderbuffer

A hiz surface must be supplied to the hardware when rendering to a depth
buffer with hiz. There are three potential places to store that surface:
    1. Allocate a larger intel_region for the depthbuffer, and let the
       region's tail be the hiz surface.
    2. Allocate a separate intel_region for hiz, and store it as
       brw_context state.
    3. Allocate a separate intel_region for hiz, and store it in
       intel_renderbuffer.

We choose method 3.

Method 1 has not been chosen due to future complications it might cause
when requesting a DRI drawable's depth buffer attachment from X.

Method 2 has not been chosen because storing the hiz region apart from
the depth region makes lazy hiz/depth resolves difficult to implement.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chad Versace <chad@chad-versace.us>