drm/modes: Make width-mm/height-mm check in of_get_drm_panel_display_mode() mandatory

All users of this function require width-mm/height-mm DT property to be
present per their DT bindings, make width-mm/height-mm check mandatory.
It is generally a good idea to specify panel dimensions, so userspace
can configure e.g. scaling accordingly.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dmitry Osipenko <digetx@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
To: dri-devel@lists.freedesktop.org
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220411004728.68203-1-marex@denx.de