drm/i915/fence: Separate timeout mechanism for awaiting on dma-fences

As the timeout mechanism has grown more and more complicated, using
multiple deferred tasks and more than doubling the size of our struct,
split the two implementations to streamline the simpler no-timeout
callback variant.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180115090643.26696-2-chris@chris-wilson.co.uk