drm/armada: allow armada_drm_plane_work_queue() to silently fail

Avoid printing an error message when armada_drm_plane_work_queue() is
unable to get the vblank (eg, because we're doing a modeset.)  Continue
to report the failure to the caller, so the caller can handle this.

Move the error message into armada_ovl_plane_update().

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>