drm/{i915, xe}/stolen: make insert_node, area_address, area_size optional

drm/{i915, xe}/stolen: make insert_node, area_address, area_size optional

Since the stolen memory hooks are function pointers, make some of them
optional instead of having to define them for xe.

insert_node, area_address, and area_size are only needed on platforms
not supported by xe.

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Link: https://patch.msgid.link/0dbb460e8bd1df29df98862d08fcdfda03912673.1764930576.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>