drm/i915: Don't call gen8_fbc_sw_flush() on chv

CHV doesn't have FBC, so don't go calling gen8_fbc_sw_flush() on it.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Add a FIXME comment while at it that we should rework this a
lot more.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>