drm/i915: Drop -Wstringop-overflow

drm/i915: Drop -Wstringop-overflow

-Wstringop-overflow 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 i915's forced extra warnings, preparing for the
upcoming change and avoiding build warnings to show up.

Fixes: 2250c7ead8ad ("drm/i915: enable W=1 warnings by default")
References: https://lore.kernel.org/all/45ad1d0f-a10f-483e-848a-76a30252edbe@paulmck-laptop/
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240112154912.1775199-1-lucas.demarchi@intel.com
(cherry picked from commit 05ae67d95bade8b7facd5612baea21c12d243149)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>