drm/nouveau/bar: move bar1 initialisation into its own function

drm/nouveau/bar: move bar1 initialisation into its own function

BAR2 being done for practical reasons, this is just for consistency.

Flushes have been added after the write to bind the instance block,
as later commits will reveal the need for them.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>