drm/nouveau/nvif: protect waits against GPU falling off the bus

drivers/gpu/drm/nouveau/dispnv04/dac.c

drivers/gpu/drm/nouveau/dispnv04/hw.c

drivers/gpu/drm/nouveau/dispnv50/base507c.c

drivers/gpu/drm/nouveau/dispnv50/core507d.c

drivers/gpu/drm/nouveau/dispnv50/corec37d.c

drivers/gpu/drm/nouveau/dispnv50/disp.c

drivers/gpu/drm/nouveau/dispnv50/ovly827e.c

drivers/gpu/drm/nouveau/include/nvif/device.h

drivers/gpu/drm/nouveau/include/nvif/timer.h

drivers/gpu/drm/nouveau/nvif/Kbuild

drivers/gpu/drm/nouveau/nvif/timer.c