drm/i915: Skip adding the request to the signal tree is complete

Enabling the interrupt for the signaler takes a finite amount of time (a
few microseconds) during which it is possible for the request to
complete. Check afterwards and skip adding the request to the signal
rbtree if it complete.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170608111405.16466-3-chris@chris-wilson.co.uk