drm/i915/stolen: convert intel_stolen_node into a real struct of its own

drm/i915/stolen: convert intel_stolen_node into a real struct of its own

i915_gem_stolen.h simply defines intel_stolen_node as drm_mm_node. Make
struct intel_stolen_node an actual struct of its own right, and embed
struct drm_mm_node inside. This allow better unification between i915
and xe.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/36762f611566d81427e702369f4e8207ead5f26c.1758732183.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>