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.