Fix the following error if FB_DDC=n

Kernel: arch/x86_64/boot/bzImage is ready  (#1)
  Building modules, stage 2.
  MODPOST 229 modules
WARNING: "fb_ddc_read" [drivers/video/nvidia/nvidiafb.ko] undefined!
make[1]: *** [__modpost] Error 1
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Cc: James Simmons <jsimmons@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>