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 <karol.wachowski@linux.intel.com>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231028155936.1183342-7-stanislaw.gruszka@linux.intel.com