drm/i915: Use LRI to update the semaphore registers

The bspec was recently updated to remove the ability to update the
semaphore using the MI_SEMAPHORE_BOX command, the ability to wait upon
the semaphore value remained. Instead the advice is to update the
register using the MI_LOAD_REGISTER_IMM command. In cursory testing,
semaphores continue to function - the question is whether this fixes
some of the deadlocks where the semaphore registers contained stale
values?
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel J Blueman <daniel@quora.org>
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -558,12 +558,9 @@ update_mboxes(struct intel_ring_buffer *ring,
 	    u32 seqno,
 	    u32 mmio_offset)
 {
-	intel_ring_emit(ring, MI_SEMAPHORE_MBOX |
-			      MI_SEMAPHORE_GLOBAL_GTT |
-			      MI_SEMAPHORE_REGISTER |
-			      MI_SEMAPHORE_UPDATE);
-	intel_ring_emit(ring, seqno);
+	intel_ring_emit(ring, MI_LOAD_REGISTER_IMM(1));
 	intel_ring_emit(ring, mmio_offset);
+	intel_ring_emit(ring, seqno);
 }
 
 /**