drm/amdgpu: fix CPDMA hang in PRT mode for VEGA20

Fix CPDMA hang in PRT mode for both VEGA10 and VEGA20

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Tested-by: Yukun.Li <yukun1.li@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>