drm/nv50-: prepare for attaching a SOR to multiple heads

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

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

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

diff --cc drivers/gpu/drm/nouveau/nv50_display.c
Simple merge