drm/ttm: Use drm_memcpy_from_wc for TTM bo moves

Use fast wc memcpy for reading out of wc memory for TTM bo moves.

Cc: Dave Airlie <airlied@gmail.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com> #v4
Link: https://lore.kernel.org/r/20210602083818.241793-6-thomas.hellstrom@linux.intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20210602083818.241793-6-thomas.hellstrom@linux.intel.com