drm/i915: Move the execbuffer objects list from the stack into the tracker

Instead of passing around the eb-objects hashtable and a separate object
list, we can include the object list into the eb-objects structure for
convenience.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>