drm: Move dma functions into drm_legacy.h

Also drop the unneeded EXPORT_SYMBOL and sprinkle drm_legacy_ prefixes
where missing.

v2: Drop the confusing _core_ and drop extern, both suggested by
David.

Cc: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>