drm/i915/skl: Provide a gen9 specific init_render_ring()

WaDisableAsyncFlipPerfMode isn't listed for SKL and
INSTPM_FORCE_ORDERING is MBZ so let's make a gen9 specific render init
function.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>