drm/i915/gt: Flush before changing register state

Flush; invalidate; change registers; invalidate; flush.

Will this finally work on every device? Or will Baytrail complain again?

On the positive side, we immediately see the benefit of having hsw-gt1 in
CI.

Fixes: ace44e13 ("drm/i915/gt: Clear CACHE_MODE prior to clearing residuals")
Testcase: igt/gem_render_tiled_blits # hsw-gt1
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>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210125220247.31701-1-chris@chris-wilson.co.uk


(cherry picked from commit d30bbd62b1bfd9e0a33c3583c5a9e5d66f60cbd7)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>