[media] SOLO6x10: Fix video headers on certain hardware

On certain platforms a sequence of dma_map_sg() and dma_unmap_sg()
discards data previously stored in the buffers. Build video headers
only after the DMA is completed.

Signed-off-by: Krzysztof Ha?asa <khalasa@piap.pl>
[hans.verkuil@cisco.com: fix merge problems due to the recent solo sg_table changes]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>