Revert "drm: of: Lookup if child node has panel or bridge"

Commit '80253168 ("drm: of: Lookup if child node has panel or
bridge")' attempted to simplify the case of expressing a simple panel
under a DSI controller, by assuming that the first non-graph child node
was a panel or bridge.

Unfortunately for non-trivial cases the first child node might not be a
panel or bridge.  Examples of this can be a aux-bus in the case of
DisplayPort, or an opp-table represented before the panel node.

In these cases the reverted commit prevents the caller from ever finding
a reference to the panel.

This reverts commit '80253168

 ("drm: of: Lookup if child node has
panel or bridge")', in favor of using an explicit graph reference to the
panel in the trivial case as well.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220420231230.58499-2-bjorn.andersson@linaro.org