drm/rockchip: lvds: 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.

Remove rockchip_lvds.h's own HIWORD_UPDATE macro, and replace all
instances of it with hw_bitfield.h's FIELD_PREP_WM16 macro, which gives
us more error checking.

For the slightly-less-trivial case of the 2-bit width instance, the
results were checked during development to match all possible input
values (0 to 3, inclusive).

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>