drm/fsl-dcu: fix opencoded use of drm_panel_*

Use drm_panel_get_modes() to access modes.
This has a nice side effect to simplify the code.

drm_panel_get_modes() may return a negative value if
for example panel is NULL. This is a small change
compared to before, but really what we want.

v2:
- Add more info to changelog (Stefan)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Alison Wang <alison.wang@nxp.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190804201637.1240-6-sam@ravnborg.org