RGBM on textures have no effect on cubemap if they are already assigned to the faces

[yaustar]

Repro steps:

1. Add RGBM encoded PNG cubemap faces to the editor (see that RGBM is not ticked on the asset in Editor)
2. Create a cubemap
3. Add faces
4. Change face textures to RGBM
5. See that no change to the visuals of the cubemap are seen

Workaround: Remove faces from cubemap. Change faces textures to RGBM. Readd faces to RGBM