drm/nv50/kms: move framebuffer wrangling out of common code

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

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

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

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