radv: Preliminary refactoring for enabling transfer queues

Various refactoring in some command buffer helper functions. This will make it easier to support transfer queues.

The main purpose of this MR is reducing code duplication for emitting some simple packets such as writing data, waiting on a memory region and emitting a fence. This is needed because SDMA has a different packet format and it's easier to support if there is less code duplication. Additionally there are also some minor changes for using const more, etc.