drm/amd/display: Program viewport when source pos changes for DCN20 hw seq

[Why]
For medium updates that change nothing but the source rect position
the viewport doesn't change on DCN20.

We're missing the check for the position update bit that was there in
the DCN10 hardware sequencer.

[How]
Check the position bit along with the scaling bit like we were doing
with DCN20.

We shouldn't actually hit a case where context != current_state in
our programming/commit model but guard against it anyway since it was
guarded for the other bits.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>