Add a function to find a connector based on a fwnode.

This will be used by the new drm_connector_oob_hotplug_event()
function which is added by the next patch in this patch-set.

Changes in v2:
- Complete rewrite to use a global connector list in drm_connector.c
  rather then using a class-dev-iter in drm_sysfs.c

Changes in v3:
- Add forward declaration for struct fwnode_handle to drm_crtc_internal.h
  (fixes warning reported by kernel test robot <lkp@intel.com>)
Tested-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20210817215201.795062-4-hdegoede@redhat.com