[media] vpif: Fix compilation with allmodconfig

When vpif is compiled as module, those errors happen:

ERROR: "vpif_lock" [drivers/media/platform/davinci/vpif_display.ko] undefined!
ERROR: "vpif_lock" [drivers/media/platform/davinci/vpif_capture.ko] undefined!

That's because vpif_lock symbol is not exported.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>