drm/ttm: fix kerneldoc of ttm_bo_create

The kerneldoc header of ttm_bo_create() was referring to another
(nonexisting) function and had a few obsolete or incorrect arguments.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>