drm/rockchip: psr: Remove flush by CRTC

It is not used anymore after last changes and it was not even correct to
begin with as it assumed a 1:1 relation between a CRTC and encoder,
while in fact a CRTC can be attached to multiple encoders.

Signed-off-by: Tomasz Figa <tfiga@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: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423105003.9004-28-enric.balletbo@collabora.com