drm/i915: Eliminate drm_gem_object_lookup during relocation

As we provide a list of all objects that will be accessed from the
batchbuffer, we can build a lut of the handles associated with those
objects for this invocation and use that to avoid the overhead of
looking up those objects again for every relocation.

The cost of building and searching a small hash table is much less than
that of acquiring a spinlock, searching a radix tree and manipulating an
atomic refcnt per relocation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>