drm/amdgpu: cleanup VCN IB generation

Start to use amdgpu_bo_create_reserved v2.

v2:
Fix missing pointer init to NULL.
Remove extra new lines.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Tested-and-Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>