drm/i915: Prevent integer overflow when validating the execbuffer

Commit 2549d6c2

 removed the vmalloc used for temporary storage of the
relocation lists used during execbuffer. However, our use of vmalloc was
being protected by an integer overflow check which we do want to
preserve!

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>