drm/xe: Remove "graphics tile" from kernel doc

Avoid using "graphics tile" to refer to GT since it's ambiguous: it's
**part** of a tile and there's also "media gt". In several places it's
documented as "GT structure", so just follow it.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250103001111.331684-3-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>