media: imx7: csi: Remove imx7_csi_get_upstream_endpoint()

The function imx7_csi_get_upstream_endpoint() is not necessary for
imx7. First, the imx7 CSI only receives from the CSI mux, so much of
the code in there is pointless. Second, it is only used to determine
whether the CSI mux has selected the CSI-2 input or the parallel input.
This can be accomplished much more simply by getting the function type
of selected input entity to the CSI mux.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>