drm/amdgpu: Rename kfd_bo_va_list to kfd_mem_attachment

This name is more fitting, especially for the changes coming next to
support multi-GPU systems with proper DMA mappings. Cleaned up the code
and renamed some related functions and variables to improve readability.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oak Zeng <Oak.Zeng@amd.com>
Acked-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>