drm/i915/gt: Invalidate as we write the gen7 breadcrumb

Still the saga of the hsw live_blt incoherency continues. While it did
seem that the invalidate before the breadcrumb had improved the mtbf,
nevertheless live_blt still failed. Mika's next idea was to pull the
invalidate-stall into the breadcrumb write itself.

References: 860afa08 ("drm/i915/gt: Flush gen7 even harder")
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/20191113151956.32242-1-chris@chris-wilson.co.uk