In the next patch, we will use the irq_posted technique for another
engine interrupt, rather than use two members for the atomic updates, we
can use two bits of one instead. First, we need to update the
breadcrumbs to use the new common engine->irq_posted.

v2: Use set_bit() rather than __set_bit() to ensure atomicity with
respect to other bits in the mask
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170124151805.26146-1-chris@chris-wilson.co.ukReviewed-by: NMika Kuoppala <mika.kuoppala@intel.com>