drm/i915: Invalidate the guc ggtt TLB upon insertion

Move the GuC invalidation of its ggtt TLB to where we perform the ggtt
modification rather than proliferate it into all the callers of the
insert (which may or may not in fact have to do the insertion).

v2: Just do the guc invalidate unconditionally, (afaict) it has no impact
without the guc loaded on gen8+
v3: Conditionally invalidate the guc - just in case that register has
not been validated for other modes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170112110050.25333-1-chris@chris-wilson.co.uk