drm/amd/display: Refine update flags usage in update_dchubp_dpp

- Only update DPP clock if it's a full update.
- Program viewport on position change. This caused SLS regressions.

Signed-off-by: Andrew Jiang <Andrew.Jiang@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>