drm/amd/display: sspl: cleanup filter code

[Why & How]
Remove unused filters and functions
Add static to limit scope

Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Jun Lei <jun.lei@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>