ARM/dma-mapping: take the bus limit into account in __dma_alloc

The DMA coherent allocator needs to take bus limits into account for
picking the zone that the memory is allocated from.

Reported-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Roger Quadros <rogerq@ti.com>