drm/bridge: sii902x: fix missing reference to mclk clock

Add devm_clk_get call to retrieve reference to master clock.

Fixes: ff578163

 ("drm/bridge: sii902x: Implement HDMI audio support")

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1563811560-29589-2-git-send-email-olivier.moysan@st.com