r600g: enable streamout only on 2.14 or latter kernel

r600g: enable streamout only on 2.14 or latter kernel

The kernel streamout support was supposed to get into 3.3 along
the tiling change and thus use the same kernel version bump of
2.13 to report userspace that streamout register were supported.

This is not what happen. So as streamout kernel support did not
bump the kernel driver version, rely on kernel 2.14 version bump
to know if streamout is enabled or not. Which means you need at
least 3.4 kernel.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>