i965: Set width, height, and tiling properly for null render targets.

i965: Set width, height, and tiling properly for null render targets.

The HW docs say that the width and height of null render targets need
to match the width and height of the corresponding depth and/or
stencil buffers, and that they need to be marked as Y-tiled.  Although
leaving these values at 0 doesn't seem to cause any ill effects, it
seems wise to follow the documented requirements.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>