drm/fb-helper: Push locking into restore_fbdev_mode_atomic|legacy

Same game as with the panning function, use drm_modeset_lock_all for
legacy paths, and a proper acquire ctx w/w mutex dance for atomic.

Cc: John Stultz <john.stultz@linaro.org>
Cc: Thierry Reding <treding@nvidia.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170704151833.17304-8-daniel.vetter@ffwll.ch