[media] vpif_display: protect dma_queue by a spin_lock

The dma_queue list is accessed by both the interrupt handler and by normal
code. It needs to be protected by a lock to prevent possible list corruption.
Corruption has been observed in 'real-life' conditions. Adding this lock made
it go away.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>