nouveau: Silence a warning at -Og

nouveau: Silence a warning at -Og

   ../src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp: In member function ‘void nv50_ir::AlgebraicOpt::handleCVT_EXTBF(nv50_ir::Instruction*)’:
   ../src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp:2238:57: warning: ‘offset’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Just set it to 0 to handle the corner case.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8724>