drm/i915/selftests: Pass around intel_context for sseu

Combine the (i915_gem_context, intel_engine) into a single parameter,
the intel_context for convenience and later simplification.

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/20190426163336.15906-4-chris@chris-wilson.co.uk