drm/i915/gt: One more flush for Baytrail clear residuals

CI reports that Baytail requires one more invalidate after CACHE_MODE
for it to be happy.

Fixes: ace44e13

 ("drm/i915/gt: Clear CACHE_MODE prior to clearing residuals")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210119110802.22228-1-chris@chris-wilson.co.uk