dma-direct: always allow dma mask <= physiscal memory size

This way an architecture with less than 4G of RAM can support dma_mask
smaller than 32-bit without a ZONE_DMA.  Apparently that is a common
case on powerpc.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>