dma-coherent: fix rmem_dma_device_init regression

My recent bug fix introduced another bug, which caused rmem_dma_device_init
to always fail, as rmem->priv is never set to anything.

This restores the previous behavior, calling dma_init_coherent_memory()
whenever ->priv is NULL.

Fixes: d35b0996

 ("dma-coherent: fix dma_declare_coherent_memory() logic error")
Reported-by: Roy Pledge <roy.pledge@nxp.com>
Tested-by: Roy Pledge <roy.pledge@nxp.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>