drm/amd/display: Add RCG helper functions

[why & how]
Add standard RCG helpers based on DCCG spec

Reviewed-by: Daniel Miess <daniel.miess@amd.com>
Reviewed-by: Muhammad Ahmed <ahmed.ahmed@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>