fbdev: propagate result of fb_videomode_from_videomode()

fb_videomode_from_videomode() may fail, but of_get_fb_videomode()
silently covers this fact. Instead, trow the error code to the
caller.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>