video: fbdev: s3c-fb: return -ENOMEM on framebuffer_alloc() failure

Fix error code from -ENOENT to -ENOMEM.

Acked-by: Jingoo Han <jingoohan1@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>