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: 2250c7ea ("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