video: fbdev: controlfb: remove function prototypes part #2

Reorder code a bit and then remove no longer needed function
prototypes.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200324134508.25120-6-b.zolnierkie@samsung.com