drm/i915/dp: Add link training debug and error printing helpers

Add functions for printing link training debug and error messages, both
to prepare for the next patch, which downgrades an error to a debug
message if the sink is disconnected and to remove some code duplication.

v2: (Ville)
- Always print the connector prefix.
- Preserve the drm_dbg_kms() debug category.
v3:
- Keep printing the name of functions calling the helpers. (Jani)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v2)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230510103131.1618266-9-imre.deak@intel.com