drm/i915: Don't force serialisation on marking up execlists irq posted

Since we coordinate with the execlists tasklet using a locked schedule
operation that ensures that after we set the engine->irq_posted we
always have an invocation of the tasklet, we do not need to use a locked
operation to set the engine->irq_posted itself.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517121007.27224-12-chris@chris-wilson.co.uk