drm: Fix up drm_atomic_state_helper.[hc] extraction

I've misplaced two functions by accident:
- drm_atomic_helper_duplicate_state is really part of the
  resume/suspend/shutdown device-wide helpers.
- drm_atomic_helper_legacy_gamma_set is part of the legacy ioctl
  compat helpers.

Move them both back.

Fixes: 9ef8a9dc ("drm: Extract drm_atomic_state_helper.[hc]")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181128100728.4674-1-daniel.vetter@ffwll.ch