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: NMichel Thierry <michel.thierry@intel.com>
Reviewed-by: NAkash Goel <akash.goel@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>