drm/i915/selftests: Convert eviction selftests to gt/ggtt

Convert the test code to work directly on what it needs rather than
going through the top-level i915.

This enables another natural usage for for_each_engine(.., gt, ..).

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022094726.3001-9-tvrtko.ursulin@linux.intel.com