backend-drm: fix drm find wrong connector

In a multi-GPU environment, different cards may contain connectors with the
same ID, and drm_head_find_by_connector just use the connector_id to find
the connector, it may find the wrong connector.
Fix this by find the connector based on the drm device and connector id.

Signed-off-by: Zhou Liang <174381115@qq.com>