drm/i915: unify gen6/gen8 rps irq enable/disable

The GEN6 and GEN8 versions differ only in the PM IIR and IER register
addresses and that on GEN8 we need to keep the
GEN8_PMINTR_REDIRECT_TO_NON_DISP PM interrupt unmasked. Abstract away
these 3 things in the GEN6 versions of the helpers and use them
everywhere.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>