fb: put framebuffer drivers back under the "frame buffer devices" menu

The config FB_PRE_INIT_FB entry in drivers/video/Kconfig pushes all entries
below it out of the menuconfig selection. Fix this.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>