Pre-Vega HW always interprets the alpha for this format as unsigned,
so we have to implement a fixup to do the sign correctly for signed
formats.
v2: Improve indexing mess.
CC: 18.0 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106480
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(Backport of 3d4d388e39 "radv: Fix up 2_10_10_10 alpha sign.")
9a4b915517