drm/i915/selftests: Remove redundant live_context for eviction

We just need the context image from the logical state to force eviction
of many contexts, so simplify by avoiding the GEM context container.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201223154509.14155-1-chris@chris-wilson.co.uk