drm/msm: fix opencoded use of drm_panel_*

Use the function drm_panel_get_modes().

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Alexios Zavras <alexios.zavras@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Allison Randal <allison@lohutok.net>
Cc: Enrico Weigelt <info@metux.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190804201637.1240-7-sam@ravnborg.org