drm/i915: don't read or write GEN6_PMIIR on Gen 5

The register doesn't exist on Gen 5.

v2: Simplify checks since pm_iir is always 0 on Gen 5 (Chris)

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>