drm/i915/guc: Call invalidate after changing the vfunc

To make this operation a bit cleaner, we should make sure that the HW
can catch up by calling the new implementation right away.
Note that currently we're only touching the vfunc at module load time
(before GuC is even loaded), so this shouldn't cause any functional
changes.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171213221352.7173-4-michal.winiarski@intel.com