drm/amd/display: Handle dml allocation failure to avoid crash

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Handle dml allocation failure to avoid crash [Why] In the case where a dml allocation fails for any reason, the current state's dml contexts would no longer be valid. Then subsequent calls dc_state_copy_internal would shallow copy invalid memory and if the new state was released, a double free would occur. [How] Reset dml pointers in new_state to NULL and avoid invalid pointer (cherry picked from commit bcafdc61529a48f6f06355d78eb41b3aeda5296c)