drm/atomic: implement drm_atomic_helper_commit_tail for runtime_pm users

The current drm_atomic_helper_commit_tail helper works only if the CRTC is
accessible, and documents an alternative implementation that is supposed to
be used if that happens.

That implementation is then duplicated by some drivers. Instead of
documenting it, let's implement an helper that all the relevant users can
use directly.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/a8f92dc70048bab746e94dadd1c23200626aff60.1500555652.git-series.maxime.ripard@free-electrons.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>