fbdev: fbmem: make fb_show_logo_line return the end instead of the height

In preparation for allowing centering of the bootup logo, make
fb_show_logo_line return where the next free framebuffer line is,
instead of returning the height of the shown logo.

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