i965/fs: Handle negating immediates on MADs when propagating saturates

MADs don't take immediate sources, but we allow them in the IR since it
simplifies a lot of things. I neglected to consider that case.

Fixes: 4009a9ea ("i965/fs: Allow saturate propagation to propagate
                      negations into MADs.")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103616


Reported-and-Tested-by: Ruslan Kabatsayev <b7.10110111@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit a05af1f7)