drm/xe/ads: Use flexible-array

Zero-length arrays are deprecated and flexible arrays should be
used instead: https://www.kernel.org/doc/html/v6.9-rc7/process/deprecated.html#zero-length-and-one-element-arrays

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@inria.fr>
Closes: https://lore.kernel.org/r/202405051824.AmjAI5Pg-lkp@intel.com/
Fixes: dd08ebf6

 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240506141917.205714-1-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
(cherry picked from commit ee728423

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