drm/amd/display: Simplify underscan and ABM commit

[Why]
Underscan and ABM are connector properties but require updates
to DC stream state.  Previously, on updates to these properties
the affected stream and all its planes were committed.
This is unnecessary; only a few fields on the stream need
to be changed.

[How]
If scaling or ABM have changed, change the stream and
create a stream update with those changes.  Call
DC with only those fields.

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>