mesa: Replace the LA16_UNORM packed formats with one array format.

mesa: Replace the LA16_UNORM packed formats with one array format.

The array format is what the GL API wants (and we made a mistake in
the format returned for texbos on big-endian!), and it's exactly what
the gallium-side PIPE_FORMAT_L16A16 is.  The only downside is that
dri_util tries to fall back to sampling RG16 using LA16, which doesn't
have a match for big-endian any more.  No HW drivers supported A16L16
anyway.

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