media: ti-vpe: cal: support 8 DMA contexts

The current driver only ever needs 2 DMA contexts (one per PHY), but we
need to use more of the 8 contexts to add support for multiple streams.

Change the code so that we allocate DMA contexts as needed, which at
this time is 1 per PHY, but could be up to 8.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>