drm/i915/gen9: fix PIPE_CONTROL flush for VS_INVALIDATE

On GEN9+ per specification a NULL PIPE_CONTROL needs to be emitted
before any PIPE_CONTROL command with the VS_INVALIDATE flag set.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>