mesa: Keep a computed value for dual source blend func with each buffer.

mesa: Keep a computed value for dual source blend func with each buffer.

The i965 driver needed this as well for hardware setup, so instead of
duplicating the logic, just save it off.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>