drm/i915: batch_obj vm offset must be u64

Otherwise it can overflow in 48-bit mode, and cause an incorrect
exec_start.

Before commit 5f19e2bf ("drm/i915: Merged
the many do_execbuf() parameters into a structure"), it was already an u64.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>