There's no point in enabling PSR when the panel doesn't support it.

This also avoids a problem when PSR gets enabled when a CRTC is being
disabled, because sometimes in that situation the DSP_HOLD_VALID_INTR
interrupt on which we wait will never arrive. This was observed on
RK3288 with a panel without PSR (veyron-jaq Chromebook).

It's very easy to reproduce by running the kms_rmfb test in IGT a few
times.

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/1474639600-30090-2-git-send-email-tomeu.vizoso@collabora.com