drm/i915: don't try to load GuC fw on pre-gen9

This avoids some bad register writes and generally feels more correct
than unconditionally trying to redirect interrupts and such.

References: https://bugs.freedesktop.org/show_bug.cgi?id=91777
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>