drm/i915/bios: make the aux channel macros private to the vbt parser

Hide the aux channel macros in intel_vbt_defs.h now that their use has
been abstracted in intel_bios_port_aux_ch().

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181115105237.1237-3-jani.nikula@intel.com