Fix function prototype to match its actual usage and implementation.

drivers/gpu/drm/ttm/ttm_bo_util.c:341:10: error: symbol 'ttm_io_prot' redeclared with different type (originally declared at include/drm/ttm/ttm_bo_driver.h:911) - incompatible argument 1 (different signedness)
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc:	David Airlie <airlied@linux.ie>
Signed-off-by: Dave Airlie <airlied@redhat.com>