drm/amd/display: Check null pointers before used

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Check null pointers before used [WHAT & HOW] Poniters, such as dc->clk_mgr, are null checked previously in the same function, so Coverity warns "implies that "dc->clk_mgr" might be null". As a result, these pointers need to be checked when used again. This fixes 10 FORWARD_NULL issues reported by Coverity.