drm/i915/bdw: Use the GEN8 SRM when qeueing a flip

Comment from Ben: It's a bit unclear whether we need this dance still
on bdw.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>