i965: Pull format conversion logic out of brw_depthbuffer_format.

i965: Pull format conversion logic out of brw_depthbuffer_format.

brw_depthbuffer_format is not very reusable at the moment, since it
uses global state (ctx->DrawBuffer) to access a particular depth buffer.

For HiZ on Broadwell, I need a function which simply converts the
formats.  However, at least one existing user of brw_depthbuffer_format
really wants the existing interface.  So, I've created a new function.

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