drm/i915: Use intel_display wherever possible

drm/i915: Use intel_display wherever possible

Use struct intel_display wherever possible in intel_display_debug_fs.c
to reduce the use of drm_i915_private.
While at it do the opportunistic display->platform.xx replacement.

--v2
-Rebase

--v3
-Don't use inline to_intel_display [Jani]
-Don't use display debug fs as subject prefix [Jani]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250212074542.3569452-2-suraj.kandpal@intel.com