CVE-2023-52913 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)