drm/i915: PSR VLV: Add single frame update.

According to spec: "In PSR HW or SW mode, SW set this bit before writing
registers for a flip. It will be self-clear when it gets to the PSR
active state."

Some versions of spec mention that this is needed when in
"Persistent mode" but define it as same as "SW mode". Since this
fix the page flip case let's assume this is exactly what we need.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>