gallium: fix a warning

gallium: fix a warning

On some platforms (like Win64), unsigned long is 32-bit, so the first
cast doesn't do anything, and the compiler complains about an implicit
cast to a smaller type. So let's cast to an uintptr_t instead first,
as that's large enough on all platforms.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Brian Paul <brianp@vmware.com>