drm/sun4i: rgb: add missing calls to drm_panel_{prepare,unprepare}

If the enable-gpios property of a simple panel in device tree is set,
the GPIO is not toggled on/off because of missing calls to
drm_panel_prepare and drm_panel_unprepare.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>