drm/amdgpu/umsch: reinitialize write pointer in hw init

Otherwise the old one will be used during GPU reset.
That's not expected.
Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/umsch_mm_v4_0.c

@@ -225,6 +225,8 @@ static int umsch_mm_v4_0_ring_start(struct amdgpu_umsch_mm *umsch)
 
 	WREG32_SOC15(VCN, 0, regVCN_UMSCH_RB_SIZE, ring->ring_size);
 
+	ring->wptr = 0;
+
 	data = RREG32_SOC15(VCN, 0, regVCN_RB_ENABLE);
 	data &= ~(VCN_RB_ENABLE__AUDIO_RB_EN_MASK);
 	WREG32_SOC15(VCN, 0, regVCN_RB_ENABLE, data);