drm/amd/display: HUBP/HUBBUB register programming fixes

[Why]
- Need to change interface function signature / add an enum
  to reflect the available register field values

[How]
- Add a new enum and modify existing functions to use it instead
  of bool

Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>