v3dv/meta-copy: add uintptr_t casting to avoid warning

Without it, on the rpi4 (and any 32-bit SO) we would get a warning
about wrong sizes.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>
diff --git a/src/broadcom/vulkan/v3dv_meta_copy.c b/src/broadcom/vulkan/v3dv_meta_copy.c
index 032e4c9..42e0e3b 100644
--- a/src/broadcom/vulkan/v3dv_meta_copy.c
+++ b/src/broadcom/vulkan/v3dv_meta_copy.c
@@ -1767,7 +1767,7 @@
                          VkAllocationCallbacks *alloc)
 {
    V3DV_FROM_HANDLE(v3dv_device, device, _device);
-   struct v3dv_bo *bo = (struct v3dv_bo *) pobj;
+   struct v3dv_bo *bo = (struct v3dv_bo *)((uintptr_t) pobj);
    v3dv_bo_free(device, bo);
 }