drm/amd/display: Add double buffering to dcn20 OCSC

[Why]
When rapidly adjusting colour properties (e.g. brightness), screen tearing was observed.
This was due to overwritten values in OCSC registers. In dcn10, this issue had been fixed by
implementing double buffering by alternating OCSC modes.

[How]
Alternate which OCSC registers are used by switching modes each time.
This double buffers the CSC writes.

Signed-off-by: Noah Abradjian <noah.abradjian@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>