drm/mxsfb: fix opencoded use of drm_panel_*

Use the drm_panel_get_modes() function.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Marek Vasut <marex@denx.de>
Acked-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20190804201637.1240-8-sam@ravnborg.org