drm/i915: object size needs to be u64

In a 48b world, users can try to allocate buffers bigger than 4GB; in
these cases it is important that size is a 64b variable.

v2: Drop the warning about bind with size 0, it shouldn't happen anyway.
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>