drm/amd/pm: Remove many unnecessary NULL values

Ther are many pointers assigned first, which need not to be initialized, so
remove the NULL assignment.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>