plug-ins, psd: fix #12745 invalid merged image when selecting cmyk
This seems to fix the problem of invalid merged image data in psd images.
@dnovomesky Could you check if this fixes loading in kimageformats?
I don't like having to increment channels twice, I might have another look at possible changes. We also seem to load this incorrectly. Some cmyk psd merged images need to be loaded as rgb, while others as cmyk? Needs more investigation in a separate issue.