drm/amd/display: Fix DML validation of simple vs native 422 modes

[Why]
We're always validating DML with simple 422 DSC even if native 422 DSC
is in use.

[How]
Use the mode configuration from the timing.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Bindu Ramamurthy <bindur12@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>