drm: bridge/dw_hdmi: fixed codec style

Using a local struct pointer to reduce one level of indirection
makes the code slightly more readable.

Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>