drm/i915: kill a no longer necessary BUG_ON

i915_gem_object_move_to_active can handle zero seqno for us now.
And not emitting a request is not fatal here - we'll try to emit
a new one if we have to wait for some rendering to complete.

In case this assumption ever gets accidentally broken, there's already
a BUG_ON to catch it in i915_do_wait_request.

So just silently ignore ENOMEM here instead of screwing up the whole
drm.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem.c

@@ -3827,7 +3827,6 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
 	 * wait on when trying to clear up gtt space).
 	 */
 	seqno = i915_add_request(dev, file_priv, ring);
-	BUG_ON(seqno == 0);
 	for (i = 0; i < args->buffer_count; i++) {
 		struct drm_gem_object *obj = object_list[i];
 		obj_priv = to_intel_bo(obj);