dma-buf: Use atomic_fetch_add() for the context id

Now that atomic64_fetch_add() exists we can use it to return the base
context id, rather than the atomic64_add_return(N) - N concoction.

Suggested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Maciej Patelczyk <maciej.patelczyk@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200513074809.18194-2-chris@chris-wilson.co.uk