i965: Refactor Gen8 depth packet emission.

i965: Refactor Gen8 depth packet emission.

The existing code followed the vtable function signature, which is not a
great fit: many of the parameters are unused, and the function still
inspects global state, making it less reusable.

This patch refactors the depth buffer packet emission code into a new
function which takes exactly the parameters it needs, and which uses no
global state.  It then makes the existing vtable function call the new
one.

Ideally, we would remove the vtable function, and clean up that
interface.  But that can happen once HiZ is working.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>