nouveau: Fix a couple of "foo may be used uninitialized' compiler warnings

nouveau: Fix a couple of "foo may be used uninitialized' compiler warnings

These are all new false positives with gcc6.

In nouveau_compiler.c: gcc6 no longer assumes that passing a pointer
to a variable into a function initialises that variable.

In nv50_ir_from_tgsi.cpp op and mode are not set if there are 0
enabled dst channels, this never happens, but gcc cannot know this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>