drm/i915: use intel_bios_is_port_present()

Don't access i915->vbt.ddi_port_info[] directly.

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/42544944ce505826335bab30cc76e135581229be.1579270868.git.jani.nikula@intel.com