drm: of: introduce drm_of_find_panel_or_bridge

Many drivers have a common pattern of searching the OF graph for either an
attached panel or bridge and then finding the DRM struct for the panel
or bridge. Also, most drivers need to handle deferred probing when the
DRM device is not yet instantiated. Create a common function,
drm_of_find_panel_or_bridge, to find the connected node and the
associated DRM panel or bridge device.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
[seanpaul dropped extern from drm_of.h]
Signed-off-by: Sean Paul <seanpaul@chromium.org>