drm/nouveau/kms/nv50: fix double dma_fence_put() when destroying plane state

When the atomic support was added to nouveau, the DRM core did not do this.

However, later in the same merge window, a commit (drm/fence: add in-fences
support) was merged that added it, leading to use-after-frees of the fence
object.

Cc: stable@vger.kernel.org	[4.10+]
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>