drm/i915/ringbuffer: Flush writes before RING_TAIL update

Be paranoid and make sure we flush any and all writes out of the WCB
before performing the UC mmio to update the RING_TAIL. (An UC write
should itself be enough to do the flush, hence the paranoia here.) Quite
infrequently, we see problems where the GPU seems to overshoot the
RING_TAIL and so executes garbage, hence the speculation.

References: https://bugs.freedesktop.org/show_bug.cgi?id=111598
References: https://bugs.freedesktop.org/show_bug.cgi?id=111417
References: https://bugs.freedesktop.org/show_bug.cgi?id=111034


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190909113018.13300-1-chris@chris-wilson.co.uk