d3d12: Initialize local_resource member mapped in constructor.

Fix defect reported by Coverity Scan.

Uninitialized scalar field (UNINIT_CTOR)
uninit_member: Non-static class member mapped is not initialized in this
constructor nor in any functions that it calls.
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <mesa/mesa!8193>

src/gallium/drivers/d3d12/d3d12_resource.cpp

@@ -622,7 +622,8 @@ synchronize(struct d3d12_context *ctx,
 /* A wrapper to make sure local resources are freed and unmapped with
  * any exit path */
 struct local_resource {
-   local_resource(pipe_screen *s, struct pipe_resource *tmpl)
+   local_resource(pipe_screen *s, struct pipe_resource *tmpl) :
+      mapped(false)
    {
       res = d3d12_resource(d3d12_resource_create(s, tmpl));
    }