drm/vc4: hdmi: Fix HDMI monitor detection in polled mode

When vc4_hdmi_connector_detect() was called in
connector_status_connected state it incorrectly cleared the
hdmi_monitor flag, leading to no audio on RPi3.

Fix this by clearing hdmi_monitor only when the hpd check
indicated no connection or if reading the edid failed.

Signed-off-by: Matthias Reichl <hias@horus.com>