drm/amd/display: Miss register MST encoder cbs

It is to fix: MST display failed to resume from S3

Need to properly setup MST encoder cbs. Otherwise drm_device
encoder doesn't register its own cbs, leading to NULL
encoder->funcs in drm_atomic_helper_resume().

Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>