drm/fb-helper: Extract _legacy kms functions

The goal is to push all the kms locking down into these separate
_atomic and _legacy functions, so that we can correctly pass the
acquire ctx into all atomic drivers. Instead of playing games with
hidden ctx in mode_config.acquire_ctx. All the fbdev state will be
protected by a new fbdev private lock that Thierry is working on.

This here is just prep by creating a clean split between atomic and
legacy paths, which also simplifies the control flow a bit.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403083304.9083-16-daniel.vetter@ffwll.ch