drm/i915/dp: use drm_dp_phy_name() for logging

Drop the local intel_dp_phy_name() function, and replace with
drm_dp_phy_name(). This lets us drop a number of local buffers.

v2: Rebase

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220912132313.2774603-1-jani.nikula@intel.com