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

[dinghao.liu at zju.edu.cn]

> 
> > For security, I will release this pointer only on error paths in this function.
> 
> Do you tend to release objects (which are referenced by pointers)?
> 

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. Thank you for reminding me!

Regards,
Dinghao