drm/i915: PSR HSW: update after enabling sprite.

On the current structure HSW doesn't support PSR with sprites enabled
but sprites can be enabled after PSR was enabled what would cause
user to miss screen updates.

v2: move it to update_plane.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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>