Move internal declarations to drm_legacy.h and add drm_legacy_*() prefix
to all legacy functions.

[airlied: add a bit of an explaination to drm_legacy.h]
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>