drm/bridge: analogix_dp: Only wait for panel ACK on PSR entry

We currently wait for the panel to mirror our intended PSR state
before continuing on both PSR enter and PSR exit. This is really
only important to do when we're entering PSR, since we want to
be sure the last frame we pushed is being served from the panel's
internal fb before shutting down the soc blocks (vop/analogix).

This patch changes the behavior such that we only wait for the
panel to complete the PSR transition when we're entering PSR, and
to skip verification when we're exiting.

Cc: Stéphane Marchesin <marcheu@chromium.org>
Cc: Sonny Rao <sonnyrao@chromium.org>
Signed-off-by: zain wang <wzz@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.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: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180309222327.18689-7-enric.balletbo@collabora.com