drm/i915: break intel_infoframe_flags into _enable and _frequency

This will allow us to disable an infoframe without changing its
frequency.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>