Cycles: Disable HIP RT by default in Blender 4.5

At the moment there are two main usability issues that make it hard to
recommend to enable HIP RT by default:
- Dramatically increased memory usage during BVH construction on
  high poly meshes compared to BVH2 (#136174)
  - This issue can be fixed by using the "balanced" HIP RT BVH, but
  it requires a HIP RT update that won't make it into 4.5 (!136622)
- Many Blender and GPU driver crashes when modifying objects in the
  viewport. #140763, #140738, #139013, #138043

Pull Request: https://projects.blender.org/blender/blender/pulls/140794

intern/cycles/blender/addon/properties.py

@@ -1612,7 +1612,7 @@ class CyclesPreferences(bpy.types.AddonPreferences):
     use_hiprt: BoolProperty(
         name="HIP RT",
         description="HIP RT enables AMD hardware ray tracing on RDNA2 and above",
-        default=True,
+        default=False,
     )
 
     use_oneapirt: BoolProperty(