drm/atomic-helper: Don't call atomic_update_plane when it stays off
It's a silly thing to do and surprises driver writers. Most likely
this did already blow up for exynos.
It's also a silly thing to change plane state when it's off, but fbdev
is silly (it does an unconditional modeset over all planes). And
userspace can be evil. So I think we need this.
With this check in the helpers we can remove the one in i915 code for
the same conditions (becuase ->crtc iff ->fb).