drm/amd/display: Return DMUB_STATUS_OK when autoload unsupported

[Why]
Not having support for autoload isn't an error. If the DMUB firmware
doesn't support it then don't return DMUB_STATUS_INVALID.

[How]
Return DMUB_STATUS_OK when ->is_auto_load_done is NULL.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>