imx-drm: parallel-display: Fix drm_panel support

Call drm_panel_enable/disable in reaction to encoder_commit
and encoder_disable callbacks.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>