drm/xe: Fix build bug for GCC 11

drm/xe: Fix build bug for GCC 11

Building drivers/gpu/drm/xe/xe_gt_pagefault.c with GCC 11 results
in the following build errors:

./include/linux/fortify-string.h:57:33: error: writing 16 bytes into a region of size 0 [-Werror=stringop-overflow=]
   57 | #define __underlying_memcpy     __builtin_memcpy
      |                                 ^
./include/linux/fortify-string.h:644:9: note: in expansion of macro ‘__underlying_memcpy’
  644 |         __underlying_##op(p, q, __fortify_size);                        \
      |         ^~~~~~~~~~~~~
./include/linux/fortify-string.h:689:26: note: in expansion of macro ‘__fortify_memcpy_chk’
  689 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
      |                          ^~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/xe/xe_gt_pagefault.c:340:17: note: in expansion of macro ‘memcpy’
  340 |                 memcpy(pf_queue->data + pf_queue->tail, msg, len * sizeof(u32));
      |                 ^~~~~~
In file included from drivers/gpu/drm/xe/xe_device_types.h:17,
                 from drivers/gpu/drm/xe/xe_vm_types.h:16,
                 from drivers/gpu/drm/xe/xe_bo.h:13,
                 from drivers/gpu/drm/xe/xe_gt_pagefault.c:16:
drivers/gpu/drm/xe/xe_gt_types.h:102:25: note: at offset [1144, 265324] into destination object ‘tile’ of size 8
  102 |         struct xe_tile *tile;
      |                         ^~~~

Fix these by removing -Wstringop-overflow from drm/xe builds.

Closes: https://lore.kernel.org/all/45ad1d0f-a10f-483e-848a-76a30252edbe@paulmck-laptop/
Fixes: 7a8bc11782d3 ("drm/xe: Enable W=1 warnings by default")
Suggested-by: Stephen Rothwell <sfr@rothwell.id.au>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
[ This particular warning is broken on GCC11. In future changes it will
  be moved to the normal C flags in the top level Makefile (out of
  Makefile.extrawarn), but accounting for the compiler support. Just
  remove it out of xe's forced extra warnings for now ]
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>