drm/vc4: dpi: Ensure a default format is selected

In a couple of error/incomplete configuration cases, the
DPI_FORMAT bits wouldn't get set.

Enforce our RGB888 default in all these cases.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20220613144800.326124-11-maxime@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>