drm: Check for connection_mutex in drm_select_eld

drm_select_eld should check for mode_config.connection_mutex as
well as mode_config.mutex: We need that since this function checks
conector->encoder links.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
[danvet: Pimp commit message slightly.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>