drm/ttm: fix the warning for hit_low and evict_low

fix the below warning messages:
ttm/ttm_bo.c:1098: warning: Function parameter or struct member 'hit_low' not described in 'ttm_bo_swapout_walk'
ttm/ttm_bo.c:1098: warning: Function parameter or struct member 'evict_low' not described in 'ttm_bo_swapout_walk'

Cc: Maarten Lankhorst <dev@lankhorst.se>
Cc: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Maarten Lankhorst <dev@lankhorst.se>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20250423042442.762108-1-sunil.khatri@amd.com