drm/rockchip: Don't key off vblank for psr

Instead of keying off vblank for psr, just flush every time
we get an atomic update. This ensures that cursor updates
will properly disable psr (without turning vblank on/off),
and unifies the paths between fb_dirty and atomic psr
enable/disable.

Reviewed-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>