drm/amd/display: Add private data type for RCG

[why & how]
Add private data types for better RCG control

Reviewed-by: Chris Park <chris.park@amd.com>
Reviewed-by: Yihan Zhu <yihan.zhu@amd.com>
Signed-off-by: Hansen Dsouza <hansen.dsouza@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>