drm/i915: Move intel_aux_ch() to intel_bios.c

drm/i915: Move intel_aux_ch() to intel_bios.c

From ICL onwards all the DDI/TypeC ports - even working in HDMI mode -
need to know their corresponding AUX channel, so move the corresponding
helper to a common place.

No functional change.

v4:
- Fix 'no space is necessary after a cast' checkpatch warn.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181101140427.31026-2-imre.deak@intel.com