[media] videobuf2-dma-contig: make cookie() return a pointer to dma_addr_t
dma_addr_t may not fit into void* on some architectures. To be safe, make
vb2_dma_contig_cookie() return a pointer to dma_addr_t and dereference it
in vb2_dma_contig_plane_paddr() back to dma_addr_t.