drm/imx: Fix memory leak in imx_pd_connector_get_modes

Avoid leaking the display mode variable if of_get_drm_display_mode
fails.

Fixes: 76ecd9c9

 ("drm/imx: parallel-display: check return code from of_get_drm_display_mode()")
Addresses-Coverity-ID: 1443943 ("Resource leak")
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://lore.kernel.org/r/20220108165230.44610-1-jose.exposito89@gmail.com