drm/dp: use DRM_DEBUG_DP() instead of drm_dbg for logging

We have a wrapper for a reason.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190121112758.10978-1-jani.nikula@intel.com