FROMLIST: dma-buf: dma-heap: Keep track of the heap device struct

Keep track of the heap device struct.

This will be useful for special DMA allocations
and actions.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Link: https://lore.kernel.org/lkml/20201110034934.70898-7-john.stultz@linaro.org/
Bug: 170887642
Change-Id: I55b8df5fd9fd46795a8f0171318895297d2dd379