video: theadorable: Use RGB565 for BMP blitting

At present this uses RGB555 format for blitting to a display. Sandbox uses
565 and that seems to be more normal for BMP as well. Update the code
accordingly and add a test.

Note that this likely breaks the theadorable board so we may need to
discuss supporting both formats.

Signed-off-by: Simon Glass <sjg@chromium.org>