drm/i915: Don't write IVB_FBC_RT_BASE

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: 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>