drm/amd/display: Check null pointers before using dc->clk_mgr

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Check null pointers before using dc->clk_mgr [WHY & HOW] dc->clk_mgr is null checked previously in the same function, indicating it might be null. Passing "dc" to "dc->hwss.apply_idle_power_optimizations", which dereferences null "dc->clk_mgr". (The function pointer resolves to "dcn35_apply_idle_power_optimizations".) This fixes 1 FORWARD_NULL issue reported by Coverity.