drm/bridge: dw-hdmi: Restore audio when setting a mode

When setting a new display mode, dw_hdmi_setup() calls
dw_hdmi_enable_video_path(), which disables all hdmi clocks, including
the audio clock.

We should only (re-)enable the audio clock if audio was already enabled
when setting the new mode.

Without this patch, on RK3288, there will be HDMI audio on some monitors
if i2s was played to headphone when the monitor was plugged.
ACER H277HU and ASUS PB278 are two of the monitors showing this issue.

Signed-off-by: Cheng-Yi Chiang <cychiang@chromium.org>
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191008102145.55134-1-cychiang@chromium.org