[media] videobuf-dma-contig: use gfp_t for GFP flags

Sparse complains if about using unsigned long.
videobuf-dma-contig.c:47:67: warning: restricted gfp_t degrades to integer
videobuf-dma-contig.c:47:65: warning: incorrect type in argument 2 (different base types)
videobuf-dma-contig.c:47:65:    expected restricted gfp_t [usertype] gfp_mask
videobuf-dma-contig.c:47:65:    got unsigned long

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>