i965: Stop making a pointless region for DRI2 to just throw it away.

i965: Stop making a pointless region for DRI2 to just throw it away.

I noticed that we were doing this while changing the DRI3 path to not use
regions, which involved changing the signature of
intel_update_winsys_renderbuffer_miptree() this way.

v2: Replace my comment with Chad's version.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net> (v1)
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>