OMAP2: avoid descending into disabled framebuffer dirs

Rather than always add the omap2 dirs to the build list (and thus
force everyone to generate a useless built-in.o), bind the dirs to
their relevant kconfig symbol.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/omap2/Makefile

 obj-$(CONFIG_OMAP2_VRAM) += vram.o
 obj-$(CONFIG_OMAP2_VRFB) += vrfb.o
 
-obj-y += dss/
-obj-y += omapfb/
+obj-$(CONFIG_OMAP2_DSS) += dss/
+obj-$(CONFIG_FB_OMAP2) += omapfb/
 obj-y += displays/