drm/i915/selftests: Mark up sole accessor to ctx->vm as being protected

In the selftests, where we are accessing a private ctx from within the
confines of a single test, we know that the ctx->vm pointer is static
and bounded by the lifetime of the test. We can use a simple helper to
provide the RCU annotations to keep sparse happy.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107221201.30497-1-chris@chris-wilson.co.uk