drm/i915: Initial selftests for exercising eviction

Very simple tests to just ask eviction to find some free space in a full
GTT and one with some available space.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-41-chris@chris-wilson.co.uk