drm/bridge: add of_drm_get_bridge_by_endpoint()

drm/bridge: add of_drm_get_bridge_by_endpoint()

drm_of_find_panel_or_bridge() is widely used, but many callers pass NULL
into the @panel or the @bridge arguments, thus making a very partial usage
of this rather complex function.

Besides, the bridge returned in @bridge is not refcounted, thus making this
API unsafe when DRM bridge hotplug will be introduced.

Solve both issues for the cases of calls to drm_of_find_panel_or_bridge()
with a NULL @panel pointer by adding a new function that only looks for
bridges (and is thus much simpler) and increments the refcount of the
returned bridge.

The new function is identical to drm_of_find_panel_or_bridge() except it:

 - handles bridge refcounting: uses of_drm_find_and_get_bridge() instead of
   of_drm_find_bridge() internally to return a refcounted bridge
 - is simpler to use: just takes no @panel parameter, returns the pointer
   in the return value instead of a double pointer argument
 - has a simpler implementation: it is equal to
   drm_of_find_panel_or_bridge() after removing the code that becomes dead
   when @panel == NULL

Also add this function to drm_bridge.c and not drm_of.c because it returns
bridges only.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patch.msgid.link/20260511-drm-bridge-alloc-getput-panel_or_bridge-v6-2-f61c9e498b3f@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>