drm/i915/selftests: Use a full emulation of a user ppgtt context

To test eviction from a ppgtt, we just want a ppgtt i.e. something other
than the Global GTT which is shared and used by the kernel for HW
features like fencing and scanout. However, we also need it to pass
!i915_is_ggtt() and the simplest way is to emulate a full user context
rather than the internal kernel context that is used for the GGTT.

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/20180719194746.19111-1-chris@chris-wilson.co.uk