drm/amd/display: Adjust dmub outbox notification enable

[Why]
Currently driver enables dmub outbox notification before oubox ISR is
registered. During boot scenario, sometimes dmub issues hpd outbox
message before driver registers ISR and those messages are missed.

[How]
Enable dmub outbox notification after outbox ISR is registered. Also,
restructured outbox enable code to call from dm layer and renamed APIs.
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn31/dcn31_hwseq.c

@@ -197,10 +197,6 @@ void dcn31_init_hw(struct dc *dc)
 		}
 	}
 
-	/* Enables outbox notifications for usb4 dpia */
-	if (dc->res_pool->usb4_dpia_count)
-		dmub_enable_outbox_notification(dc->ctx->dmub_srv);
-
 	/* we want to turn off all dp displays before doing detection */
 	dc->link_srv->blank_all_dp_displays(dc);