V4L/DVB (6997): Replace a very dirty hack on videobuf for a clean wait_event

In order to videobuf_iolock to work, mmap_mapper should be called first.
Otherwise, an OOPS is generated.

On some cases, .mmap file handler used to took some time to be called. On those
situations, mmap_mmapper() were called after iolock.

This patch properly waits for mmap_mapper to be called, otherwise generating an
error.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>