drm/i915: Select DRM_VGEM for igt

igt/prime_vgem (and others) depends upon VGEM so automatically select it when
enabling i915 debugging.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1468059777-10205-1-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Matthew Auld <matthew.auld@intel.com>