drm/i915: Track cursor changes as frontbuffer tracking flushes

We treat other plane updates in the same fashion. Spotted because
Rodrigo kept reporting a bug in the PSR code where the frontbuffer was
eternally stuck with a dirty cursor bit set.

The psr testcase should have caught this, but that i-g-t is kaputt.
Rodrigo is signed up to fix that.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by-and-Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>