drm/amd/display: Return error code on DSC atomic check failure

[Why&How]
We were not returning -EINVAL on DSC atomic check fail. Add it.

Fixes: 71be4b16

 ("drm/amd/display: dsc validate fail not pass to atomic check")
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>