drm/i915/gt: Flush gen7 even harder

live_blt is still failing on hsw, showing the hallmark of incoherency.
Since we are fairly certain that the interrupt is after the seqno is
visible, the other possibility is that the seqno is before the writes to
memory are flushed. Throw in an TLB invalidate before the breadcrumb as
we are reasonably confident that forces a CS stall.

References: f9228f76 ("drm/i915/gt: Try an extra flush on the Haswell blitter")
References: https://bugs.freedesktop.org/show_bug.cgi?id=112147


Testcase: igt/i915_selftest/live_blt
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/20191112160941.23969-1-chris@chris-wilson.co.uk