drm/i915: unify gen6/gen8 pm irq helpers

The helpers to enable/disable PM IRQs for GEN6 and GEN8 are the same
except for the PM interrupt mask register, so abstract away this
register in the GEN6 versions and use these 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>