drm/fb-helper: Split dpms handling into legacy and atomic paths

Like with panning and modesetting, and like with those, stick with
simple drm_modeset_locking_all for the legacy path, and the full
atomic dance for atomic drivers.

This means a bit more boilerplate since setting up the atomic state
machinery is rather verbose, but then this is shared code for 30+
drivers or so, so meh.

After this patch there's only the LUT/cmap path which is still using
drm_modeset_lock_all for an atomic driver. But Peter is already
locking into reworking that, so I'll leave that code as-is for now.

v2: Squash in patches from Maarten to unify all the various atomic
paths into just one atomic update function for fbdev overall. On top
do one s/restore_fbdev_mode/restore_fbdev_mode_atomic/ so that we have
all-atomic callchains after the first check.

Cc: Peter Rosin <peda@axentia.se>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.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/20170704151833.17304-10-daniel.vetter@ffwll.ch