We use nuking instead of render tracking on IVB+, so there's
no point in writing IVB_FBC_RT_BASE.

v2: Drop the IVB_FBC_RT_BASE write too
v3: Move the SNB stuff elsewhere, leaving only IVB+ here
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>