drm/amd/display: Fix Coverity Issues

[Why]
Coverity discovers holes in logic that needs to be addressed for
improved code integrity.

[How]
Address issues found by coverity without changing the actual logic.

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Chris Park <Chris.Park@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>