drm/bridge: analogix_dp: Add analogix_dp_psr_supported

So users know whether PSR should be enabled or not.

Cc: Yakir Yang <ykk@rock-chips.com>

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1474639600-30090-1-git-send-email-tomeu.vizoso@collabora.com