drm: Remove transitional helpers

With armada the last bigger driver that realistically needed these to
convert from legacy kms to atomic is converted. These helpers have
been broken more often than not the past 2 years, and as this little
patch series shows, tricked a bunch of people into using the wrong
helpers for their functions.

Aside: I think a lot more drivers should be using the device-level
drm_atomic_helper_shutdown/suspend/resume helpers and related
functions. In almost all the cases they get things exactly right.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-16-daniel.vetter@ffwll.ch