drm/rockchip: Fix up bug in psr state machine

The ->set() callback would always be called when transitioning
from FLUSH->DISABLE since we assign state to psr->state right
above the skip condition.

Reported-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>