drm/i915/dp: convert DP test debugfs to struct intel_display

drm/i915/dp: convert DP test debugfs to struct intel_display

Prefer struct intel_display over struct drm_i915_private. Do some
drive-by logging conversions to kms category.

Observe that i915_displayport_test_active_write() was using the wrong
type for m->private, but it has worked because struct drm_i915_private
has struct drm_device at offset 0.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4d29cf43c7067e910fdf1127afcc35dd558b4b0b.1726833193.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>