drm: Move sg functions into drm_legacy.h

Also sprinkle the drm_legacy_ prefix where missing.

v2: Drop extern from function declarations and include "drm_legacy.h"
in drm_scatter.c, spotted by David.

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