drm/i915: Add wait_for in init_ring_common

I have seen a number of "blt ring initialization failed" messages
where the ctl or start registers are not the correct value. Upon further
inspection, if the code just waited a little bit, it would read the
correct value. Adding the wait_for to these reads should eliminate the
issue.
Signed-off-by: NSean Paul <seanpaul@chromium.org>
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -290,9 +290,9 @@ static int init_ring_common(struct intel_ring_buffer *ring)
 			| RING_REPORT_64K | RING_VALID);
 
 	/* If the head is still not zero, the ring is dead */
-	if ((I915_READ_CTL(ring) & RING_VALID) == 0 ||
-	    I915_READ_START(ring) != obj->gtt_offset ||
-	    (I915_READ_HEAD(ring) & HEAD_ADDR) != 0) {
+	if (wait_for((I915_READ_CTL(ring) & RING_VALID) != 0 &&
+		     I915_READ_START(ring) == obj->gtt_offset &&
+		     (I915_READ_HEAD(ring) & HEAD_ADDR) == 0, 50)) {
 		DRM_ERROR("%s initialization failed "
 				"ctl %08x head %08x tail %08x start %08x\n",
 				ring->name,