drm/amd/display: Proper return of result when aux engine acquire fails

[Why]
When aux engine acquire fails, we missed populating the operation_result
that describes the failure reason.

[How]
Set operation_result to new type:
AUX_CHANNEL_OPERATION_FAILED_ENGINE_ACQUIRE
in the case aux engine acquire has failed.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>