video: fbdev: controlfb: fix warnings about comparing pointer to 0

Fix warnings aboout comparing pointer to 0 reported by coccicheck:

drivers/video/fbdev/controlfb.c: WARNING comparing pointer to 0
drivers/video/fbdev/controlfb.c: WARNING comparing pointer to 0
drivers/video/fbdev/controlfb.c: WARNING comparing pointer to 0

Also while at it change return value to -ENOMEM on kzalloc() failure.

Signed-off-by: Shobhit Kukreti <shobhitkukreti@gmail.com>
Reviewed-by: Mathieu Malaterre <malat@debian.org>
[b.zolnierkie: minor fixups]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>