[Intel-gfx] GPU-DRM-i915: Delete an unnecessary check before the function call "pwm_put"

[SF Markus Elfring]

>> The pwm_put() function tests whether its argument is NULL and then
>> returns immediately. Thus the test around the call is not needed.
> 
> The compiler doesn't need it, but IMO it's useful documentation for humans.

How do you think about to extend the explicit documentation for
the affected parameters in the Linux programming interfaces?

Regards,
Markus