drm/bridge: analogix_dp: Split the platform-specific poweron in two parts

Some of the platform-specific stuff in rockchip_dp_poweron() needs to
happen before the generic code.  Some needs to happen after.  Let's
split the callback in two.

Specifically we can't start doing PSR work until _after_ the whole
controller is up, so don't set the enable until the end.

Cc: Kristian H. Kristensen <hoegsberg@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
[seanpaul added exynos change]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423105003.9004-23-enric.balletbo@collabora.com