drm/amd/display: Add NULL check for function pointer in dcn20_set_output_transfer_func

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

drm/amd/display: Add NULL check for function pointer in dcn20_set_output_transfer_func

This commit adds a null check for the set_output_gamma function pointer

in the dcn20_set_output_transfer_func function. Previously,

set_output_gamma was being checked for null at line 1030, but then it

was being dereferenced without any null check at line 1048. This could

potentially lead to a null pointer dereference error 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 at line 1048.