drm/xe/vsec: Address static checker issue

drm/xe/vsec: Address static checker issue

The callback structure should be static.

Add static attribute.

Fixes: 0c45e76fcc62 ("drm/xe/vsec: Support BMG devices")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412050418.8ecQxYXj-lkp@intel.com/
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241205183649.3120032-1-michael.j.ruhl@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>