i810: switch to vm_mmap()

Weirdness around do_mmap() in there does not rely on ->mmap_sem for
exclusion, so no need to keep it under that.  As the result, we can
turn that do_mmap() into vm_mmap().
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/gpu/drm/i810/i810_dma.c

@@ -130,11 +130,10 @@ static int i810_map_buffer(struct drm_buf *buf, struct drm_file *file_priv)
 		return -EINVAL;
 
 	/* This is all entirely broken */
-	down_write(&current->mm->mmap_sem);
 	old_fops = file_priv->filp->f_op;
 	file_priv->filp->f_op = &i810_buffer_fops;
 	dev_priv->mmap_buffer = buf;
-	buf_priv->virtual = (void *)do_mmap(file_priv->filp, 0, buf->total,
+	buf_priv->virtual = (void *)vm_mmap(file_priv->filp, 0, buf->total,
 					    PROT_READ | PROT_WRITE,
 					    MAP_SHARED, buf->bus_address);
 	dev_priv->mmap_buffer = NULL;
@@ -145,7 +144,6 @@ static int i810_map_buffer(struct drm_buf *buf, struct drm_file *file_priv)
 		retcode = PTR_ERR(buf_priv->virtual);
 		buf_priv->virtual = NULL;
 	}
-	up_write(&current->mm->mmap_sem);
 
 	return retcode;
 }