drm/i915: invalidate render cache on gen2

It looks like we also need to flush the render cache when we just
invalidate it. This fixes a regression in i-g-t/gem_tiled_blits on my
i855gm. I guess the render cache there is virtually indexed, so we
need to clean it when changing gtt mappings.

This regression has been introduce in

commit 46f0f8d1
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Apr 18 11:12:11 2012 +0100

    drm/i915: Don't set a MBZ bit in gen2/3 MI_FLUSH

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>