drm/i915: BDW PSR: Add single frame update support.

When link is in stand by and PSR exit is triggered by a primary or sprite
plane flip this mode allows only one single updated frame to be send to
display than get back to PSR immediately.

Reviewed-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>