drm/i915: Be paranoid and post the writes to stop the rings

Although the mmio are uncached and so should be flushed on every write,
be paranoid and do a mmio read after setting the ring head/tail to be
sure they have taken effect before moving on.

v2: post tail to be pleasing to the eye

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208072800.595-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

drivers/gpu/drm/i915/intel_uncore.c

@@ -1522,9 +1522,11 @@ static void gen3_stop_engine(struct intel_engine_cs *engine)
 				 engine->name);
 
 	I915_WRITE_FW(RING_HEAD(base), I915_READ_FW(RING_TAIL(base)));
+	POSTING_READ_FW(RING_HEAD(base)); /* paranoia */
 
 	I915_WRITE_FW(RING_HEAD(base), 0);
 	I915_WRITE_FW(RING_TAIL(base), 0);
+	POSTING_READ_FW(RING_TAIL(base));
 
 	/* The ring must be empty before it is disabled */
 	I915_WRITE_FW(RING_CTL(base), 0);