video: fbdev: pxafb: clear allocated memory for video modes

When parsing the video modes from DT properties, make sure to zero out
memory before using it. This is important because not all fields in the mode
struct are explicitly initialized, even though they are used later on.

Fixes: 420a4882 ("video: fbdev: pxafb: initial devicetree conversion")
Reviewed-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>