drm/i915: Use plane->state->fb instead of plane->fb in intel_plane_restore()

plane->fb is not as reliable as plane->state->fb so let's convert
intel_plane_restore() over the the new way of thinking as well.

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>