drm/i915: FBC_CONTROL2 is gen4 only

Gen2 and gen3 don't have the FBC_CONTROL2 register, so don't
touch it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>