drm/amdgpu/vcn3.0: remove old DPG workaround

Port from VCN2.5
SCRATCH2 is used to keep decode wptr as a workaround
which fix a hardware DPG decode wptr update bug for
vcn2.5 beforehand.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>