drm/xe: Mark internal gmdid mappings as const

The mapping between HW IP version and its description is const, so
mark it as such.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240116120207.1133-1-michal.wajdeczko@intel.com