i915g: Make sure that new vbo gets updated

i915g: Make sure that new vbo gets updated

Malloc likes to reuse old address as soon as possible this would cause the
new vbo buffer to get the same address as the old. So make sure we set it to
NULL when we allocate a new one. This fixes ipers which will fill up a couple
of VBO buffers per frame.

Signed-off-by: Jakob Bornecrantz <wallbraker@gmail.com>