drm/nv50: implement possible workaround for NV86 PGRAPH TLB flush hang

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

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

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

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

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

diff --cc drivers/gpu/drm/nouveau/nv50_fifo.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nv50_graph.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nv50_instmem.c
Simple merge