drm/xe: Make wedged_mode debugfs writable

The intent of this debugfs entry is to allow modification of wedging
behavior, either from IGT tests or during manual debug; it should be
marked as writable to properly reflect this.  In practice this hasn't
caused a problem because we always access wedged_mode as root, which
ignores file permissions, but it's still misleading to have the entry
incorrectly marked as RO.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Fixes: 6b8ef44c ("drm/xe: Introduce the wedged_mode debugfs")
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241002230620.1249258-2-matthew.d.roper@intel.com