drm/i915: Unwrap top level fence-array

By first unwrapping an incoming fence-array into its child fences, we
can simplify the internal branching, and so avoid triggering a potential
bug in the next patch when not squashing the child fences on the same
timeline.

It will also have the advantage of keeping the (top-level) fence arrays
out of any fence/timeline caching since these are unordered timelines
but with a random context id.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170503093924.5320-2-chris@chris-wilson.co.uk