drm/i915/gt: Turn vm off then on again for gen7 mm switch

"Have you tried switching it off and on again?"

Set the size of the mm to 0 to disable all PD cachelines, before
enabling the whole mm again. Let's see if that tricks the TLB into
reloading.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191208143648.2986669-1-chris@chris-wilson.co.uk