drm/i915: cleanup opregion asle pipestat enable

Both intel_opregion_enable_asle() and intel_enable_asle() have shrunk
considerably. Merge them together into a static function in i915_irq.c,
and rename to better reflect the purpose and the related platforms.

No functional changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>