drm/i915/fbc: Allow on unfenced surfaces, for recent gen

Only fbc1 is tied to using a fence. Later iterations of fbc are more
flexible and allow operation on unfenced frontbuffers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: "Zanoni, Paulo R" <paulo.r.zanoni@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160819155428.1670-3-chris@chris-wilson.co.uk