drm/amd/display: DCN32 null data check

[WHY & HOW]
Avoid null curve data structure used in the cm block for the potential issue.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Yihan Zhu <Yihan.Zhu@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>