drm/bridge: analogix: Don't return -EINVAL when panel doesn't support PSR

We will ignored PSR setting if panel not support it. So, in this case,
we should return from analogix_dp_enable/disable_psr() without any
error code. Let's retrun 0 instead of -EINVAL when panel not support PSR
in analogix_dp_enable/disable_psr().

Signed-off-by: zain wang <wzz@rock-chips.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1481072253-8917-1-git-send-email-wzz@rock-chips.com