dma-buf: dma-heap: export declared functions

Export the dma-buf heap functions to allow them to be used by the OP-TEE
driver. The OP-TEE driver wants to register and manage specific secure
DMA heaps with it.

Reviewed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>