drm/i915/dp: Add debugfs entry for DP phy compliance

These debugfs entry will help testapp to understand the test request
during dp phy compliance mode.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200316103759.12867-6-animesh.manna@intel.com