drm/nv20: split PFB code out of nv10_fb.c

Most functions were quite different between NV10/NV20 already, and they're
about to get even more so.

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

diff --cc drivers/gpu/drm/nouveau/Makefile
Simple merge

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

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

diff --cc drivers/gpu/drm/nouveau/nv10_fb.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nv20_fb.c
Simple merge