samples: drivers: display: distinguish mono pixel format

In exceptional cases, monochrome displays can deviate from the
general interpretation (PIXEL_FORMAT_MONO01) of a logical 1 as
pixel on (white) and 0 as pixel off (black) and instead set the
capability to PIXEL_FORMAT_MONO10. This small but significant
difference has been ignored so far, resulting in an inverse
display for those displays. In order to be technically correct
here, the buffers for a "white" background should be set to 0
instead of 1 and for "black" to 1 instead of 0.

Signed-off-by: Stephan Linz <linz@li-pro.net>