media: ti-vpe: cal: allow more than 1 source pads

CAL RX has a single sink and a single source pad. To support multiple
streams, we will have multiple source pads (up to 8, one for each
CAL context).

Change the driver to allow creating more source pads and change the code
accordingly to handle multiple source pads. We still keep
CAL_CAMERARX_NUM_SOURCE_PADS as 1, and the behavior is unchanged.

Also rename CAL_CAMERARX_PAD_SOURCE to CAL_CAMERARX_PAD_FIRST_SOURCE to
highlight that it's the first source.

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>