drm/amd/display: Modify DHCUB waterwark structures and functions

[WHY&HOW]
Converting the watermark set structure to a union and modifying some interfaces
to accommodate future usage.

Reviewed-by: Chaitanya Dhere <chaitanya.dhere@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>