drm/amd/display: Make DCN32 3dlut function available to future DCNs

[Why & How]
Make DCN32 3dlut function non-static and thus
available for future DCNs.

Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>