drm/amd/display: Remove unnecessary NULL values

The NULL initialization of the pointers assigned by kzalloc() first is
not necessary, because if the kzalloc() failed, the pointers will be
assigned NULL, otherwise it works as usual. so remove it.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>