drm/cma: Use dma_mmap_writecombine() to mmap buffer

The GEM CMA helpers uses a custom mmap implementation based on
remap_pfn_range(). While this works when the buffer DMA and physical
addresses are identical, it fails to take IOMMU into account and tries
to mmap the buffer to userspace using the DMA virtual address instead of
the physical address. This results in mapping random physical pages when
the device is behind an IOMMU.

Use the DMA mapping dma_mmap_writecombine() function instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>