drm/i915/selftests: Remember to create the fake preempt context

drm/i915/selftests: Remember to create the fake preempt context

For the fake device we have our own set of mock contexts that need to
match the real contexts we normally create. Currently this requires us
to manually instantiate them for the selftests, which I forgot.

Reported-by: Matthew Auld <matthew.william.auld@gmail.com>
Fixes: d71a137f82f3 ("drm/i915: Introduce a preempt context")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171005105927.22991-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>