drm/rockchip: dw_hdmi: Don't call platform_set_drvdata()

The Rockchip dw_hdmi driver just called platform_set_drvdata() to get
your hopes up that maybe, somehow, you'd be able to retrieve the 'struct
rockchip_hdmi' from a pointer to the 'struct device'.  You can't.  When
we call dw_hdmi_bind() the main driver calls dev_set_drvdata(), which
clobbers our setting.

Let's just remove the platform_set_drvdata() to avoid dashing people's
hopes.

Signed-off-by: Douglas Anderson <dianders@chromium.org>