i965/gen10+: Enable object level preemption.

i965/gen10+: Enable object level preemption.

Set bit when initializing context.

v3:
 - Always toggle preemption bool to false before enabling it for the
 first time, so the state gets emitted (Chris Wilson).
 - Emit end of pipe sync with PIPE_CONTROL_RENDER_TARGET_FLUSH (Ken)

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>