drm/bridge/synopsys: dw-hdmi: don't clobber drvdata

dw_hdmi shouldn't set drvdata since some drivers might need to store
it's own data there. Rework dw_hdmi in a way to return struct dw_hdmi
instead to store it in drvdata. This way drivers are responsible to
store and pass structure when needed.

Idea was taken from the following commit:
8242ecbd ("drm/bridge/synopsys: stop clobbering drvdata")

Cc: p.zabel@pengutronix.de
Cc: Laurent.pinchart@ideasonboard.com
Cc: hjc@rock-chips.com
Acked-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214200906.31509-6-jernej.skrabec@siol.net