drm/i915/selftests: Take a VM in kernel_context()

This better models where we want to go with contexts in general where
things like the VM and engine set are create parameters instead of being
set 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/20210708154835.528166-28-jason@jlekstrand.net