drm/xe: constify engine class sysfs attributes

All of the attributes, as well as the array of attributes, can be const
and placed in rodata.

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240117122044.1544174-5-jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>