fbdev: fbmem: add config option to center the bootup logo

If there are extra logos (CONFIG_FB_LOGO_EXTRA) the heights of these
extra logos are not considered when centering the first logo vertically.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>