drm/sti: hdmi use of_get_i2c_adapter_by_node interface

This change is needed to properly lock I2C bus device and driver,
which serve DDC lines. Without this change I2C bus driver module
may gone in runtime and this won't be noticed by the driver.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>