selftests: ion: Add simple test with the vgem driver

Ion is designed to be a framework used by other clients who perform
operations on the buffer. Use the DRM vgem client as a simple consumer.
In conjunction with the dma-buf sync ioctls, this tests the full attach/map
path for the system heap.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>