Blender Git Loki

UI: Datatransfer modifier: set mix factor inactive when not in use

For Customdata layer copying, interpolation with the mixfactor is only
done for certain mix modes, now set the UI inactive if the mixfactor is
not in use.

Namely, the modes are the "Above / Below Threshold" which are only used
for flags, colors and normals and mixing is not supported in these cases.

Spotted while looking into T81914.

Differential Revision: https://developer.blender.org/D9327