mesa: Redefine the RG formats as array formats.

This is the layout used in the GL API, and maps directly to PIPE
formats with no endianness trickery.  As with the LA change, this
fixes big-endian fetching from texbos.  Also cleans up some endian
shenanigans in shader images.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>