drm/vma-manager: drm_vma_node_start() constify argument

Constify argument so functions calling into this take a const argument.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171107191348.17555-2-noralf@tronnes.org

include/drm/drm_vma_manager.h

@@ -152,7 +152,7 @@ static inline void drm_vma_node_reset(struct drm_vma_offset_node *node)
  * Start address of @node for page-based addressing. 0 if the node does not
  * have an offset allocated.
  */
-static inline unsigned long drm_vma_node_start(struct drm_vma_offset_node *node)
+static inline unsigned long drm_vma_node_start(const struct drm_vma_offset_node *node)
 {
 	return node->vm_node.start;
 }