drm/i915: PIPE_CONTROL_TLB_INVALIDATE

This has showed up in several other patches. It's required for the next
context workaround.

I tested this one on its own and saw no differences in basic tests
(performance or otherwise). This patch is relatively likely to cause
regressions, hence why it's split out.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>