drm/amdgpu: Remove redundant NULL check

Fix below warnings reported by coccicheck:
./drivers/gpu/drm/amd/display/dc/clk_mgr/dcn30/dcn30_clk_mgr.c:557:2-7: WARNING: NULL check before some freeing functions is not needed.

Fixes: 4d55b0dd ("drm/amd/display: Add DCN3 CLK_MGR")
Signed-off-by: Li Heng <liheng40@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>