V4L/DVB (6292): videobuf_core init always require callback implementation

In the past, videobuf_queue_init were used to initialize PCI DMA videobuffers.
This patch renames it, to avoid confusion with the previous kernel API, doing:
	s/videobuf_queue_init/void videobuf_queue_core_init/

Also, the operations is now part of the function parameter. The function will
also add a test if this is defined, otherwise producing BUG.

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