i915/gem/selftests: Assign the VM at context creation in igt_shared_ctx_exec

We want to delete __assign_ppgtt and, generally, stop setting the VM
after context creation.  This is the one place I could find in the
selftests where we set a VM after the fact.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-29-jason@jlekstrand.net