drm/i915/dp: refactor aux_ch_name()

Convert aux_ch_name() to a helper that prints a string to a caller
provided buffer, and use it to get more consistent aux channel
debugs. Now that all users of aux_ch_name() are in intel_dp_aux.c, we
can make it static too.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230922105645.3991066-1-jani.nikula@intel.com