radv: Fix up 2_10_10_10 alpha sign.

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 18.1 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106480


Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 3d4d388e)