mesa: Replace MESA_FORMAT_L8A8/A8L8 UNORM/SNORM/SRGB with an array format.

mesa: Replace MESA_FORMAT_L8A8/A8L8 UNORM/SNORM/SRGB with an array format.

The array format is what the GL API wants (fixing texbos on
big-endian), and matches directly to gallium's corresponding array
format.  The only driver exposing A8L8 was radeon/r200 in big-endian,
where the HW's underlying format was trying to read as array and we
needed to flip things around to make our packed format come out right
(note that while the radeon format tables had both AL and LA,
ChooseTextureFormat would only pick one of them based on endianness).

v2: Don't make r200/radeon use endian swaps.
v3: Rebase on dropping the r200 _be/_le format table removal patch
v4: reword commit message to explain why we can drop both formats
    from radeon.

Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)