drm: rp1: rp1-dpi: Add "rgb_order" property (to match VC4 DPI)

[njhollinghurst]

As on VC4, the OF property overrides the order implied by media bus format. Only 4 of the 6 possible orders are supported. New add-on hardware designs should not rely on this "legacy" feature.

This is is an attempt to deal with #6505.

Not all combinations of DRM pixel format, bus format and rgb_order have been tested yet.

[njhollinghurst]

I think this is correct, my only reticence (and the reason it's still a draft) is code size.
There might well be a smaller or cleaner way to achieve the permutation of input (shift,mask,scaling).
Alternatively, if we drop GRB, BRG support for RGB565_1X16, RGB666_1X18 there might be a simpler version that permutes output shifts (though BGR666 will still need asymmetrical settings).