drm/i915: Protect older gen against intel_gt_init_powersave()

In the middle of intel_gt_init_powersave() we have an if-chain that ends
with a universal else clause to read gen6+ registers. Older platforms
like Pineview that end up here do not like those registers and may even
OOPS whilst reading them!

Fixes: 3ea9a80132 ("drm/i915: Perform static RPS frequency setup ...")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470132927-1821-1-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>