drm/i915/ringbuffer: EMIT_INVALIDATE after switch context

The recommend procedure was to switch contexts (and mm) then invalidate
the TLBs. Make it so.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181207090213.14352-2-chris@chris-wilson.co.uk