drm/nouveau/clk/gm20b: Fix memory leak in gm20b_clk_new()

[dinghao.liu at zju.edu.cn]

> > I just found that clk is referenced by pclk in this function. When clk is freed,
> > pclk will be allocated in gm20b_clk_new_speedo0(). Thus we should not release clk
> > in this function and there is no bug here.
> 
> Can there be a need to release a clock object after a failed gk20a_clk_ctor() call?
> 

I think this mainly depends on pclk pointer. It seems that the caller of 
gm20b_clk_new() always expects pclk to be allocated unless it returns -ENOMEM,
which means kzalloc() failed. If gk20a_clk_ctor() never returns such an error 
code, we may need not to release this clock object.

Regards,
Dinghao