drm/amd/display: Send DMCU messages only if FW loaded

[Why]
Some DMCU messages were being sent in cases where
there was no DMCU FW at all, which resulted in some wait
timeouts

[How]
Delay sending some of the DMCU messages after FW
init is called and DMCU is running.

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>