[media] saa7134: drop abuse of low-level videobuf functions

saa7134-alsa used low-level videobuf functions to allocate and sync
DMA buffers. Replace this with saa7134-specific code. These functions
will not be available when we convert to vb2.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>