drm: radeon: fix overflow on 32bit systems

the type mem->start is unsigned long, so this can overflow on
32bit system, since the type addr is uint64_t.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Young Xiao <YangX92@hotmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_vm.c

@@ -946,7 +946,7 @@ int radeon_vm_bo_update(struct radeon_device *rdev,
 		bo_va->flags &= ~RADEON_VM_PAGE_WRITEABLE;
 
 	if (mem) {
-		addr = mem->start << PAGE_SHIFT;
+		addr = (u64)mem->start << PAGE_SHIFT;
 		if (mem->mem_type != TTM_PL_SYSTEM) {
 			bo_va->flags |= RADEON_VM_PAGE_VALID;
 		}