drm/i915/lvds: nuke intel_lvds_supported()

Now that intel_lvds_init() is only called for platforms that might have
LVDS, move the remaining checks to intel_setup_outputs(), again similar
to other outputs, and remove the overlapping checks.

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/20190122082307.4003-3-jani.nikula@intel.com