drm/i915/tv: only call intel_tv_init() on platforms that might have TV

With most platforms not having TV support, only call intel_tv_init() on
platforms that might actually have TV, specifically gens 3 and 4.

This puts intel_tv_init() more in line with the rest of the outputs, and
makes it slightly easier for the uninitiated to figure out which
platforms actually have what.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190122082307.4003-4-jani.nikula@intel.com