i965: Remove _NEW_DEPTH state flagging on drawbuffers change.

i965: Remove _NEW_DEPTH state flagging on drawbuffers change.

Of the places noting a _NEW_DEPTH dependency, all were already checking
for _NEW_BUFFERS if appropriate.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>