drm/amdgpu: fix gfx wptr for sdma v4

The wptr value will be shitfed when function returns.
Remove the redundant shift and clean up.

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