drm/msm: Fix hw_fence error path cleanup

In an error path where the submit is free'd without the job being run,
the hw_fence pointer is simply a kzalloc'd block of memory.  In this
case we should just kfree() it, rather than trying to decrement it's
reference count.  Fortunately we can tell that this is the case by
checking for a zero refcount, since if the job was run, the submit would
be holding a reference to the hw_fence.

Fixes: f94e6a51 ("drm/msm: Pre-allocate hw_fence")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/547088/