drm/i915: flush cursors harder

Apparently they need the same treatment as primary planes. This fixes
modesetting failures because of stuck cursors (!) on Thomas' i830M
machine.

I've figured while at it I'll also roll it out for the ivb 3 pipe
version of this function. I didn't do this for i845/i865 since Bspec
says the update mechanism works differently, and there's some
additional rules about what can be updated in which order.

Tested-by: Thomas Richter <thor@math.tu-berlin.de>
Cc: stable@vger.kernel.org
Cc:  Thomas Richter <thor@math.tu-berlin.de>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>