drm/amdgpu: Correct sdma_v4 get_wptr(v2)

the original method will change the wptr value in wb.
v2:
furthur cleanup

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