drm/msm: remove unbalanced mutex unlock

This regression stems from 0e08270a1f01 ("drm/msm: Separate locking of
buffer resources from struct_mutex").

Signed-off-by: Daniel Mack <daniel@zonque.org>
Cc: Sushmita Susheelendra <ssusheel@codeaurora.org>
Cc: Rob Clark <rclark@redhat.com>
Fixes: 0e08270a1f01 ("drm/msm: Separate locking of buffer resources from struct_mutex")
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/dsi/dsi_host.c

@@ -1036,7 +1036,6 @@ static int dsi_tx_buf_alloc(struct msm_dsi_host *msm_host, int size)
 
 		ret = msm_gem_get_iova(msm_host->tx_gem_obj,
 				priv->kms->aspace, &iova);
-		mutex_unlock(&dev->struct_mutex);
 		if (ret) {
 			pr_err("%s: failed to get iova, %d\n", __func__, ret);
 			return ret;