drm/modes: Add of_get_drm_panel_display_mode()

Add a function to get a drm_display_mode from a panel-timing
device tree subnode.

Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220227124713.39766-4-noralf@tronnes.org