drm/sun4i: backend: Use runtime_pm variant of atomic_commit_tail

During a hardware commit, the commit bit in the backend will only be
cleared if the TCON is enabled. Use the runtime_pm variant of the
atomic_commit_tail hook that makes sure that the CRTC, our TCON, is enabled
when we perform an atomic_commit.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/bde95faff2078f63e9af99c3abee5360b9050fd1.1516613040.git-series.maxime.ripard@free-electrons.com