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

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

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

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4315>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4315>