drm/i915: Fix potential context UAFs

In the Linux kernel, the following vulnerability has been resolved: drm/i915: Fix potential context UAFs gem_context_register() makes the context visible to userspace, and which point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl. So we need to ensure that nothing uses the ctx ptr after this. And we need to ensure that adding the ctx to the xarray is the *last* thing that gem_context_register() does with the ctx pointer. [tursulin: Stable and fixes tags add/tidy.] (cherry picked from commit bed4b455cf5374e68879be56971c1da563bcd90c)