drm/v3d: Plug dma_fence leak

The irq_fence and done_fence are given a reference that is never
released. The necessary dma_fence_put()s seem to have been
deleted in error in an earlier commit.

Fixes: 0b73676836b2 ("drm/v3d: Clock V3D down when not in use.")

Signed-off-by: Phil Elwell <phil@raspberrypi.org>