media: adv748x: Store the TX sink in HDMI/AFE

Both the AFE and HDMI s_stream routines (adv748x_afe_s_stream() and
adv748x_hdmi_s_stream()) have to enable the CSI-2 TX they are streaming video
data to.

With the introduction of dynamic routing between HDMI and AFE entities to
TXA, the video stream sink needs to be set at run time, and not statically
selected as the s_stream functions are currently doing.

To fix this, store a reference to the active CSI-2 TX sink for both HDMI and
AFE sources, and operate on it when starting/stopping the stream.

Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>