drm/i915/bios: rename intel_aux_ch() to intel_bios_port_aux_ch()

Conform to function naming in intel_bios.c.

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/20181115105237.1237-1-jani.nikula@intel.com