drm/amd/display: Refactor DCN4x and related code

drm/amd/display: Refactor DCN4x and related code

[why & how]
Refactor existing code related to DCN4x for better code sharing with
other modules.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Swapnil Patel <Swapnil.Patel@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>