drm/tiny: panel-mipi-dbi: Support the pixel format property

drm/tiny: panel-mipi-dbi: Support the pixel format property

Add support for these pixel format property values:
- r5g6b5, RGB565
- b6x2g6x2r6x2, BGR666

BGR666 is presented to userspace as RGB888. The 2 LSB in each color
are discarded by the controller. The pixel is sent on the wire using
8 bits per word (little endian) so the controller sees it as BGR.

RGB565 is the default if the property is not present.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240604-panel-mipi-dbi-rgb666-v4-5-d7c2bcb9b78d@tronnes.org
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>