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).

Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: dri-devel@lists.freedesktop.org
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>