drm/ingenic: Use standard drm_atomic_helper_commit_tail

By making the CRTC's .vblank_enable() function return an error when it
is known that the hardware won't deliver a VBLANK, we can drop the
ingenic_drm_atomic_helper_commit_tail() function and use the standard
drm_atomic_helper_commit_tail() function instead.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210808134526.119198-4-paul@crapouillou.net