drm/nouveau/kms: Remove field nvbo from struct nouveau_framebuffer

The buffer object stored in nvbo is also available GEM object in obj[0]
of struct drm_framebuffer. Therefore remove nvbo in favor obj[0] and
replace all references accordingly. This may require an additional cast.

With this change we can already replace nouveau_user_framebuffer_destroy()
and nouveau_user_framebuffer_create_handle() with generic GEM helpers.
Calls to nouveau_framebuffer_new() receive a GEM object.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>