drm/i915/skl: SKL FBC enablement

Enable FBC feature on Skylake

Issue: VIZ-3788
Reviewed-by: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Daisy Sun <daisy.sun@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>