drm/rockchip: dw_hdmi: store rockchip_hdmi reference in phy_data object

When using special phy handling operations we'll often need access to
the rockchip_hdmi struct.

As the chip-data that occupies the phy_data pointer initially gets
assigned to the rockchip_hdmi struct, we can now re-use this phy_data
pointer to hold the reference to the rockchip_hdmi struct and use this
reference later on.

Inspiration for this comes from meson and sunxi dw-hdmi, which are using
the same method.

changes in v3:
- reword commit message

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Zheng Yang <zhengyang@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180912124740.20343-6-heiko@sntech.de