drm/rockchip: dw_hdmi_qp: switch to FIELD_PREP_WM16 macro

The era of hand-rolled HIWORD_UPDATE macros is over, at least for those
drivers that use constant masks.

Replace this driver's HIWORD_UPDATE with the FIELD_PREP_WM16 macro from
hw_bitfield.h. While at it, disambiguate the GRF write to SOC_CON7 by
splitting the definition into the individual bitflags. This is done
because FIELD_PREP_WM16 shifts the value for us according to the mask,
so writing the mask to itself to enable two bits is no longer something
that can be done. It should also not be done anyway because it hides the
true meaning of those two individual bit flags.

HDMI output with this patch has been tested on both RK3588 and RK3576.
On the former, with both present HDMI connectors.

Reviewed-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Tested-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>