agp: use NULL instead of 0 when pointer is needed

Fixes sparse warning:

drivers/char/agp/generic.c:1217:33: warning: Using plain integer as
NULL pointer

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
CC: linux-kernel@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>