drm/amdgpu: the warning dereferencing obj for nbio_v7_4

if ras_manager obj null, don't print NBIO err data
Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Suggested-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/nbio_v7_4.c

@@ -383,7 +383,7 @@ static void nbio_v7_4_handle_ras_controller_intr_no_bifring(struct amdgpu_device
 		else
 			WREG32_SOC15(NBIO, 0, mmBIF_DOORBELL_INT_CNTL, bif_doorbell_intr_cntl);
 
-		if (!ras->disable_ras_err_cnt_harvest) {
+		if (ras && !ras->disable_ras_err_cnt_harvest && obj) {
 			/*
 			 * clear error status after ras_controller_intr
 			 * according to hw team and count ue number