samples: drivers: display: Support line alignment constrained displays

The sample app draws rectangles in the corner of the display.
On devices with the SCREEN_INFO_X_ALIGNMENT_WIDTH capability,
each draw needs to transmit a complete line. Make the sample
work on such displays (e.g. the `ls0xx` driver) by making the
rectangles take up the full width of the display.

Co-authored-by: Benjamin Cabé <kartben@gmail.com>
Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>