drm/i915: use drm->debugfs_root for creating debugfs files

Since commit 0b30d57a ("drm/debugfs: rework debugfs directory
creation v5") we should be using drm->debugfs_root instead of
minor->debugfs_root for creating debugfs files.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/ba8a2a7ec10e54b4d0a96926ef20c96e268c0b94.1753782998.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>