drm/amdgpu: avoid dereferencing a NULL pointer

Check if irq_src is NULL to avoid dereferencing a NULL pointer,
for MES ring is uneccessary to recieve an interrupt notification.
Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -416,6 +416,8 @@ int amdgpu_fence_driver_start_ring(struct amdgpu_ring *ring,
 		ring->fence_drv.gpu_addr = adev->uvd.inst[ring->me].gpu_addr + index;
 	}
 	amdgpu_fence_write(ring, atomic_read(&ring->fence_drv.last_seq));
+
+	if (irq_src)
 		amdgpu_irq_get(adev, irq_src, irq_type);
 
 	ring->fence_drv.irq_src = irq_src;
@@ -537,6 +539,7 @@ void amdgpu_fence_driver_fini(struct amdgpu_device *adev)
 			/* no need to trigger GPU reset as we are unloading */
 			amdgpu_fence_driver_force_completion(ring);
 		}
+		if (ring->fence_drv.irq_src)
 			amdgpu_irq_put(adev, ring->fence_drv.irq_src,
 				       ring->fence_drv.irq_type);
 		drm_sched_fini(&ring->sched);
@@ -574,6 +577,7 @@ void amdgpu_fence_driver_suspend(struct amdgpu_device *adev)
 		}
 
 		/* disable the interrupt */
+		if (ring->fence_drv.irq_src)
 			amdgpu_irq_put(adev, ring->fence_drv.irq_src,
 				       ring->fence_drv.irq_type);
 	}
@@ -601,6 +605,7 @@ void amdgpu_fence_driver_resume(struct amdgpu_device *adev)
 			continue;
 
 		/* enable the interrupt */
+		if (ring->fence_drv.irq_src)
 			amdgpu_irq_get(adev, ring->fence_drv.irq_src,
 				       ring->fence_drv.irq_type);
 	}