gallium: initialize ureg_dst::Invariant bit

gallium: initialize ureg_dst::Invariant bit

When this bit was added, it seems the some initialization code
was omitted by mistake.

Since stack-variables have kinda random contents, and we don't
zero initialize the whole struct in these code-paths, we end up
getting random-ish values for this bit.

Spotted by Coverity in the following CIDs:
- 1438115
- 1438123
- 1438130

Fixes: 70425bcfe63c4e9191809659d019ec4af923595d ("gallium: plumb
invariant output attrib thru TGSI")

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Jakob Bornecrantz <jakob@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>