drm/amdgpu/sdma5.2: soft reset sdma blocks before setup and start sdma

Without doing the soft reset, register mmSDMA0_GFX_RB_WPTR's value could not be
reset to 0 when sdma block resumes. That would cause the ring buffer's read and
write pointers not equal and ring test fail. So add the soft reset step.

Signed-off-by: Xiaomeng Hou <Xiaomeng.Hou@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>