[BUG] `as_column` of pandas timestamps delivers different resolution datetime depending on whether we pass a scalar or list

**Describe the bug**

```
import pandas as pd
from cudf.core.column import as_column

data = pd.Timestamp("2000-01-01")

from_scalar = as_column(data)
from_list = as_column([data])

assert from_scalar.dtype == from_list.dtype # False
```

**Expected behavior**

The resolution should be inferred consistently. Note that `cudf.Scalar(data)` infers the same (nanosecond) resolution as `as_column([data])`.