drm/{i915, xe}/stolen: rename i915_stolen_fb to intel_stolen_node
Use a more generic name than one that refers to "i915" and "fb".
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/925fd07d3f2a6115c71984f5a40a06c9eb46a539.1758732183.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>