[media] cx88: add missing alloc_ctx support

The cx88 vb2 conversion and the vb2 dma_sg improvements were developed separately and
were merged separately. Unfortunately, the patch updating drivers to the dma_sg
improvements didn't take the updated cx88 driver into account. Basically two ships
passing in the night, unaware of one another even though both ships have the same
owner, i.e. me :-)

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reported-by: Chris Lee <updatelee@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>