drm/amd/display: Add NULL check for function pointer in dcn401_set_output_transfer_func

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Add NULL check for function pointer in dcn401_set_output_transfer_func

This commit adds a null check for the set_output_gamma function pointer
in the dcn401_set_output_transfer_func function. Previously,
set_output_gamma was being checked for null, but then it was being
dereferenced without any null check. This could lead to a null pointer
dereference if set_output_gamma is null.

To fix this, we now ensure that set_output_gamma is not null before
dereferencing it. We do this by adding a null check for set_output_gamma
before the call to set_output_gamma.