[media] v4l: omap4iss: isif: Ignore VD0 interrupts when no buffer is available

The ISIF generates VD0 interrupts even when writes are disabled.
Disabling the ISIF when no buffer is available is thus not be enough, we
need to handle the situation explicitly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>