drm/i915: Unify legacy/execlists emission of MI_BATCHBUFFER_START

Both the ->dispatch_execbuffer and ->emit_bb_start callbacks do exactly
the same thing, add MI_BATCHBUFFER_START to the request's ringbuffer -
we need only one vfunc.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-20-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-10-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index fe06b58..ca941ff 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1326,9 +1326,9 @@
 	if (exec_len == 0)
 		exec_len = params->batch_obj->base.size;
 
-	ret = params->engine->dispatch_execbuffer(params->request,
-						  exec_start, exec_len,
-						  params->dispatch_flags);
+	ret = params->engine->emit_bb_start(params->request,
+					    exec_start, exec_len,
+					    params->dispatch_flags);
 	if (ret)
 		return ret;