i965: Base HW depth format setup based on MESA_FORMAT, not bpp.

i965: Base HW depth format setup based on MESA_FORMAT, not bpp.

This will make handling new formats (like actually exposing Z32F)
easier and more reliable.

v2: Remove the check for hiz buffer -- the MESA_FORMAT should really
    be giving us the value we want even for hiz.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>