venus: remove 2 redundant roundtrips
bo allocations for the below cases are after device memory allocation:
- direct non-external mappable memory allocation
- pool grow
- exportable memory allocation
For (1) and (2), the bo is for mapping, which is a pure kernel operation to happen later. So roundtrip is not needed here.
For (3), the bo is for either fd export or mapping, which are both pure kernel operations. So roundtrip is not needed either.