drm/i915: Add mock exercise for i915_gem_gtt_insert

i915_gem_gtt_insert should allocate from the available free space in the
GTT, evicting as necessary to create space.

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/20170213171558.20942-43-chris@chris-wilson.co.uk