drm/imx: Add missing DRM_FORMAT_RGB565 to ipu_plane_formats

DRM_FORMAT_RGB565 is missing from ipu_plane_formats.
The support is there, just need to make it available to userspace.

Signed-off-by: Enrico Jorns <ejo@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>