drm: Add a prefetching memcpy_from_wc

Reading out of write-combining mapped memory is typically very slow
since the CPU doesn't prefetch. However some archs have special
instructions to do this.

So add a best-effort memcpy_from_wc taking dma-buf-map pointer
arguments that attempts to use a fast prefetching memcpy and
otherwise falls back to ordinary memcopies, taking the iomem tagging
into account.

The code is largely copied from i915_memcpy_from_wc.

Cc: Daniel Vetter <[email protected]>
Cc: Christian König <[email protected]>
Suggested-by: Daniel Vetter <[email protected]>
Signed-off-by: Thomas Hellström <[email protected]>
Acked-by: Christian König <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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