i965: Replace intel_context:needs_ff_sync with intel->gen == 5.

i965: Replace intel_context:needs_ff_sync with intel->gen == 5.

Technically, needs_ff_sync was set on Gen5+, but it was only consulted
in the clipper threads and quad/lineloop decomposition code, which are
both Gen4-5 only.  So in reality it only identified Ironlake.

The named flag doesn't really clarify things, and seems like overkill.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Chris Forbes <chrisf@ijw.co.nz>
Acked-by: Paul Berry <stereotype441@gmail.com>
Acked-by: Anuj Phogat <anuj.phogat@gmail.com>