drm/i915: Force an aliasing_ppgtt test for context execution

Ensure that we minimally exercise the aliasing_ppgtt, even on a
full-ppgtt, by allocating one and similarly creating a context to use
it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-40-chris@chris-wilson.co.uk