drm/amd/display: Check for NULL when creating gamma struct

[Wjy&How] Some stress test is causing unexpected memory allocation
failure. This prevents null dereference but there will likely be problems
later, hard to gracefully handle memalloc fail for critical objects.

Signed-off-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Reza Amini <Reza.Amini@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>