drm/i915/dp: convert to drm device based logging

Prefer drm device based logging.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b1cffaa70fcc614574f2dce4461e28be7a407e30.1642769982.git.jani.nikula@intel.com