drm/i915: VLV/CHV PSR needs to exit PSR on every flush.

ON these platforms we don't have hardware tracking working for any case.
So we need to fake this on software by forcing psr to exit on every
flush.

Manual tests indicated this was needed.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>