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