drm: convert drivers to use drm_of_find_panel_or_bridge

Similar to the previous commit, convert drivers open coding OF graph
parsing to use drm_of_find_panel_or_bridge instead.

This changes some error messages to debug messages (in the graph core).
Graph connections are often "no connects" depending on the particular
board, so we want to avoid spurious messages. Plus the kernel is not a
DT validator.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
[seanpaul dropped rockchip changes since they're now obsolete]
Signed-off-by: Sean Paul <seanpaul@chromium.org>