drm/amd/display: fix dml pipe merge logic

Dml merges mpc/odm combine pipes to do calculations. This merge is
imperfect if there is a viewport overlap. This change saves pre overlap
viewport for dml use.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Reviewed-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>