drm/bridge: analogix_dp: detect Sink PSR state after configuring the PSR

Make sure the request PSR state takes effect in analogix_dp_send_psr_spd()
function, or print the sink PSR error state if we failed to apply the
requested PSR setting.

Cc: 征增 王 <wzz@rock-chips.com>
Cc: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>
[seanpaul changed timeout loop to a readx poll]
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/20180309222327.18689-2-enric.balletbo@collabora.com