Subtest names must be stable across all platforms for easier tracking.
Hence move the gen8+ check into the subtests, using igt_require. This
will auto-skip the tests on platforms where a given test doesn't apply.

Also move the assignment of the relocation_type var outside of the
fixture block. Fixtures aren't run when enumerating subtests (so that
subtests can be enumerated on any platform, even without an intel gpu).

So gcc has indeed been right with it's "potentially uninitialized" var
warning after all ...

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>