drm/ttm: Fix build with CONFIG_DEBUG_FS=n

drm/ttm: Fix build with CONFIG_DEBUG_FS=n

Move the define outside the ifdef for CONFIG_DEBUG_FS to fix the build.
This currently breaks drm kunit tests:

	$ ./tools/testing/kunit/kunit.py run --kunitconfig drivers/gpu/drm/ttm/tests/.kunitconfig
	ERROR:root:../drivers/gpu/drm/ttm/ttm_pool.c: In function ‘ttm_pool_mgr_init’:
	../drivers/gpu/drm/ttm/ttm_pool.c:1335:30: error: ‘TTM_SHRINKER_BATCH’ undeclared (first use in this function)
	 1335 |         mm_shrinker->batch = TTM_SHRINKER_BATCH;

Fixes: 22b929b25293 ("drm/ttm: Increase pool shrinker batch target")
Cc: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20250603184750.3304647-2-lucas.demarchi@intel.com