drm/ttm: use bit flag to replace allow_reserved_eviction in ttm_operation_ctx

for saving memory and more bit flag can be used in future

Signed-off-by: Roger He <Hongbo.He@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>