drm/amdgpu: make pad_ib a ring function v3

The padding depends on the firmware version and we need that for BO moves as
well, not only for VM updates.

v2: new approach of making pad_ib a ring function
v3: fix typo in macro name

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>