drm/i915: Future proof uncore_init.

Unless future specs tells otherwise we can assume future gens
inherit some stuff from the previous so let's handle
missed cases when we know tehy should't be there and assume
default equals newest one.

No functional changes.

v2: Remove useless case as pointed out by Ville.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>