ttm: Include the 'struct dev' when using the DMA API.

This makes the accounting when using 'debug_dma_dump_mappings()'
and CONFIG_DMA_API_DEBUG=y be assigned to the correct device
instead of 'fallback'.

No functional change - just cosmetic.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>