drm/vmwgfx: Remove rcu locks from user resources

In the Linux kernel, the following vulnerability has been resolved: drm/vmwgfx: Remove rcu locks from user resources User resource lookups used rcu to avoid two extra atomics. Unfortunately the rcu paths were buggy and it was easy to make the driver crash by submitting command buffers from two different threads. Because the lookups never show up in performance profiles replace them with a regular spin lock which fixes the races in accesses to those shared resources. Fixes kernel oops'es in IGT's vmwgfx execution_buffer stress test and seen crashes with apps using shared resources.