drm/i915: remove superfluous string helper include

Remove the duplicate and incorrect (uses "" instead of <>)
linux/string_helpers.h include.

Fixes: cc1338f2 ("drm/i915/xehp: Update topology dumps for Xe_HP")
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220425154754.990815-1-jani.nikula@intel.com