drm/i915: Exercise i915_vma_pin/i915_vma_insert

High-level testing of the struct drm_mm by verifying our handling of
weird requests to i915_vma_pin.

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/20170213171558.20942-35-chris@chris-wilson.co.uk