drm/i915/bxt: BXT FBC enablement

Enable FBC feature on Broxton

Issue: VIZ-3784
Signed-off-by: Daisy Sun <daisy.sun@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>