drm/radeon: fix incorrect use of the lru_lock

The BO manager has its own lock and doesn't use the lru_lock.

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