drm/amd/display: Use correct error codes

Should return -ENOMEM when allocation fails.
Also, just return the error code instead of using a variable.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>