media: ti-vpe: cal: allocate pix proc dynamically

CAL has 4 pixel processing units but the units are not needed e.g. for
metadata. As we could be capturing 4 pixel streams and 4 metadata
streams, i.e. using 8 DMA contexts, we cannot assign a pixel processing
unit to every DMA context. Instead we need to reserve a pixel processing
unit only when needed.

Add functions to reserve and release a pix proc unit, and use them in
cal_ctx_prepare/unprepare. Note that for the time being we still always
reserve a pix proc unit.

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>