Use the drm_panel_get_modes() function to get the modes.

This patch leave one test for the function pointer:
    panel->funcs->get_modes

This is used to check if the panel may have any modes.
There is no direct replacement.
We may be able to just check that drm_panel_get_modes() return > 0,
but as this is not the same functionality it is left for later.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
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/20190804201637.1240-5-sam@ravnborg.org