xen/arm: use dev_is_dma_coherent

Use the dma-noncoherent dev_is_dma_coherent helper instead of the home
grown variant.  Note that both are always initialized to the same
value in arch_setup_dma_ops.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>