drm/i915/gt: make kobj attributes const

There's no need for any of these to be mutable, constify:

drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000020 files.0
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000050 files.1
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 preempt_timeout_attr
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 timeslice_duration_attr
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 timeslice_duration_def
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 preempt_timeout_def
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 max_spin_def
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 stop_timeout_def
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 heartbeat_interval_def
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 name_attr
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 class_attr
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 inst_attr
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 mmio_attr
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 caps_attr
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 all_caps_attr
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 max_spin_attr
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 stop_timeout_attr
drivers/gpu/drm/i915/gt/sysfs_engines.o: .data	0000000000000038 heartbeat_interval_attr

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230309081645.385650-1-jani.nikula@intel.com