mesa/format_utils: Add a fast-path for RGBA to BGRA

Icecream95 requested to merge icecream95/mesa:bgra-fastpath into master

This is similar to an existing fast-path, but this is for an array source while the previous one is for an array destination.

Firefox can hit this case for WebGL when GL compositing is not used. For a WebGL sample on the Panfrost driver, the frame-rate increased from 19.4 fps to 20.6 fps, which is a 6% gain.

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