drm/i915: move Makefile display debugfs files next to display

Keep the display build lists together.

v2: Rebase

Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231026101333.875406-2-jani.nikula@intel.com