swr: fix icc compile error

ICC doesn't like the use of nullptr (std::nullptr_t) argument in
p_atomic_set.  GCC and clang don't complain.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99119
Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>
diff --git a/src/gallium/drivers/swr/swr_fence_work.cpp b/src/gallium/drivers/swr/swr_fence_work.cpp
index 3f83e61..1fd2a83 100644
--- a/src/gallium/drivers/swr/swr_fence_work.cpp
+++ b/src/gallium/drivers/swr/swr_fence_work.cpp
@@ -39,7 +39,7 @@
       work = fence->work.head.next;
       /* Immediately clear the head so any new work gets added to a new work
        * queue */
-      p_atomic_set(&fence->work.head.next, nullptr);
+      p_atomic_set(&fence->work.head.next, 0);
       p_atomic_set(&fence->work.tail, &fence->work.head);
       p_atomic_set(&fence->work.count, 0);