drm/amd/display: Check return value of drm helper

If driver fails to update update VCPI allocation during
compute_mst_dsc_configs_for_state - fail the function
by return early.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>