drm/i915: Don't set the fence number in DPFC_CTL on SNB

SNB has another register where the actual FBC CPU fence number is
stored. The documenation explicitly states that the fence number
in DPFC_CTL must be 0 on SNB. And in fact when it's not zero,
the GTT tracking simply doesn't work.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>