drm/i915/selftests: Fix live_requests for all engines

After the abandonment of i915->kernel_context and since we have started to
create per-gt engine->kernel_context, these tests need to be updated to
instantiate the batch buffer VMA in the correct PPGTT for the context used
to execute each spinner.

v2(Tejas):
  - Clean commit message - Matt
  - Add BUG_ON to match vm
v3(Tejas):
  - Fix dim checkpatch warnings

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230228044307.191639-1-tejas.upadhyay@intel.com