drm/amd/display: Clean up dynamic metadata logic

 [Why]
Code to enable DCN20 dynamic metadata feature is duplicated in two places
and was added to DCE110 enable stream.

[How]
Create DCN20 specific enable stream function for clarity, and add a hardware
sequencer function to program dynamic metadata to avoid the duplicate
code.

Signed-off-by: Julian Parkin <julian.parkin@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>