mesa/formats: Fix swizzle flipping for big-endian targets

mesa/formats: Fix swizzle flipping for big-endian targets

The swizzle defines where in the format you should look for any given
channel.  When we flip the format around for BE targets, we need to change
the destinations of the swizzles, not the sources.  For example, say the
format is an RGBX format with a swizzle of xyz1 on LE.  Then it should be
wzy1 on BE;  however, the code as it was before, would have made it 1zyx on
BE which is clearly wrong.

Reviewed-by: Iago Toral <itoral@igalia.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Cc: "10.6 10.5" <mesa-stable@lists.freedesktop.org>