drm/amd/display: Fix new dmub notification enabling in DM

[Why]
Changes from "Fix for dmub outbox notification enable" need to land
in DM or DMUB outbox notification would be disabled.

[How]
Enable outbox notification only after interrupt are enabled and IRQ
handlers registered. Any pending notification will be sent by DMUB
once outbox notification is enabled.

Fixes: ed720870 ("drm/amd/display: Fix for dmub outbox notification enable")
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>