amdgpu/dc: Fix missing null checks in amdgpu_dm.c

From smatch:
error: we previously assumed X could be null

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>