s3c-fb: change to depending on CONFIG_S3C_FB_DEV

As suggested by Marek Szyprowski, we should make the driver depend on the
configuration currently being used to build the platform device into the
kernel.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Pawel Osciak <p.osciak@samsung.com>
Cc: InKi Dae <inki.dae@samsung.com>
Cc: KyungMin Park <kyungmin.park.samsung.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>