drm/xe/uapi: Add documentation for DRM_XE_GEM_CREATE_FLAG_DEFER_BACKING

Add documentation for drm_xe_gem_create structure flag
DRM_XE_GEM_CREATE_FLAG_DEFER_BACKING.

v2: Modified to be in a more generalised way.

Signed-off-by: Priyanka Dandamudi <priyanka.dandamudi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250728043336.3319521-1-priyanka.dandamudi@intel.com


Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>