overlays: mipi-dbi-spi: width-mm and height-mm are mandatory

There was a last minute change to the binding in commit 1ecc0c09
("dt-bindings: display: panel: mipi-dbi-spi: Make width-mm/height-mm mandatory").
This was done so userspace can calculate DPI.

Update the overlay to ensure it will work with future kernels.
The driver changes won't be backported to rpi-5.15.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>