drm/i915/blt: support copying objects

We can already clear an object with the blt, so try to do the same to
support copying from one object backing store to another. Really this is
just object -> object, which is not that useful yet, what we really want
is two backing stores, but that will require some vma rework first,
otherwise we are stuck with "tmp" objects.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190810174338.19810-1-chris@chris-wilson.co.uk