drm/msm: Reuse dma_fence_release.

If we follow the typical pattern of the base class being the first
member, we can use the default dma_fence_free function.

Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: Rob Clark <robdclark@gmail.com>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>