xen/arm: use is_device_dma_coherent

xen/arm: use is_device_dma_coherent

Use is_device_dma_coherent to check whether we need to issue cache
maintenance operations rather than checking on the existence of a
particular dma_ops function for the device.

This is correct because coherent devices don't need cache maintenance
operations - arm_coherent_dma_ops does not set the hooks that we
were previously checking for existance.

Signed-off-by: Stefano Stabellini <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Acked-by: Ian Campbell <[email protected]>