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

For the legacy path we'll keep drm_modeset_lock_all, for the atomic
one we drop the use of the magic implicit context and wire it up
properly.

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/20170704151833.17304-7-daniel.vetter@ffwll.ch