drm/radeon: fix sparse warning

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_gart.c

@@ -577,7 +577,7 @@ void radeon_vm_manager_fini(struct radeon_device *rdev)
  *
  * Global and local mutex must be locked!
  */
-int radeon_vm_evict(struct radeon_device *rdev, struct radeon_vm *vm)
+static int radeon_vm_evict(struct radeon_device *rdev, struct radeon_vm *vm)
 {
 	struct radeon_vm *vm_evict;