dt-bindings: display: panel: mipi-dbi-spi: Make width-mm/height-mm mandatory

Make the width-mm/height-mm panel properties mandatory
to correctly report the panel dimensions to the OS.

Fixes: 2f3468b8

 ("dt-bindings: display: add bindings for MIPI DBI compatible SPI panels")
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>
Cc: devicetree@vger.kernel.org
To: dri-devel@lists.freedesktop.org
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220404192105.12547-1-marex@denx.de