omap2/3/4: Disable CONFIG_FB_OMAP in omap3_defconfig

Looks like CONFIG_FB_OMAP prevents somehow mounting root on MMC
at least on zoom3 for multi-omap. Disable CONFIG_FB until the
omap FB code is fixed.

This allows booting omap3_defconfig on various omaps. Tested on
2420-n8x0, 3430-n900, 3630-zoom3 and 4430-blaze. Note that n8x0
still has issues with starting user space because of TLS and
VFP.

Signed-off-by: Tony Lindgren <tony@atomide.com>