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.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>