drm/rockchip: analogix_dp: implement PSR function

Alway enable the PSR function for Rockchip analogix_dp driver. If panel
don't support PSR, then the core analogix_dp would ignore this setting.

Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>