i965: Reduce code duplication in handling of depth, stencil, and HiZ.

i965: Reduce code duplication in handling of depth, stencil, and HiZ.

This patch consolidates duplicate code in the brw_depthbuffer and
gen7_depthbuffer state atoms.  Previously, these state atoms contained
5 chunks of code for emitting the _3DSTATE_DEPTH_BUFFER packet (3 for
Gen4-6 and 2 for Gen7).  Also a lot of logic for determining the
appropriate buffer setup was duplicated between the Gen4-6 and Gen7
functions.

This refactor splits the code into three separate functions:
brw_emit_depthbuffer(), which determines the appropriate buffer setup
in a mostly generation-independent way, brw_emit_depth_stencil_hiz(),
which emits the appropriate state packets for Gen4-6, and
gen7_emit_depth_stencil_hiz(), which emits the appropriate state
packets for Gen7.

Tested using Piglit on Gen5-7 (no regressions).

v2: Re-word some comments.  Fix an assertion that incorrectly
prohibited packed depth/stencil formats on Gen6 (these are allowed
provided that HiZ is disabled).

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>