drm/bridge: anx7625: Fill in empty ELD when no connector

Speaker may share I2S with DP and .get_eld callback will be called when
speaker is playing. When HDMI wans't connected, the connector will be
null. Instead of return an error, fill in empty ELD.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Xin Ji <xji@analogixsemi.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220414090003.1806535-1-hsinyi@chromium.org