drm/nvc0-: remove nasty fifo swmthd hack for flip completion method

Not required anymore as flips are always done on the kernel's channel,
which means we can use a proper software object class instead.

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

diff --cc drivers/gpu/drm/nouveau/core/engine/fifo/nvc0.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/fifo/nve0.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nouveau_display.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nouveau_dma.h
Simple merge