media: imx7-media-csi: get csi upstream endpoint

When the upstream endpoint is neither a mux nor a CSI2 module, just get
the source pad directly upstream from the CSI.

Reported-by: Sebastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>