accel/ivpu: Make DMA allocations for MMU600 write combined

Previously using dma_alloc_wc() API we created cache coherent
(mapped as write-back) mappings.

Because we disable MMU600 snooping it was required to do costly
page walk and cache flushes after each page table modification.

With write-combined buffers it's possible to do a single write memory
barrier to flush write-combined buffer to memory which simplifies the
driver and significantly reduce time of map/unmap operations.

Mapping time of 255 MB is reduced from 2.5 ms to 500 us.

Signed-off-by: Karol Wachowski <[email protected]>
Reviewed-by: Stanislaw Gruszka <[email protected]>
Reviewed-by: Jeffrey Hugo <[email protected]>
Signed-off-by: Stanislaw Gruszka <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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