drm/i915/display: 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.

As a rule of thumb, use a local variable when there are two or more
uses, otherwise just have the single reference inline.

Drop drm/drm_file.h include where possible.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/e8268546ec2a2941a3dc43c2fdc60f678dc03fce.1753782998.git.jani.nikula@intel.com


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