drm/rcar-du: dw-hdmi: Fix compilation

Commit eea034af ("drm/bridge/synopsys: dw-hdmi: don't clobber drvdata")
broke the build with one build error and one warning. Fix both.

Cc: Archit Taneja <architt@codeaurora.org>
Cc: Jernej Skrabec <jernej.skrabec@siol.net>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Fixes: eea034af ("drm/bridge/synopsys: dw-hdmi: don't clobber drvdata")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180216154412.22876-1-maxime.ripard@bootlin.com