drm/amd/display: fix DMCU hang when going into Modern Standby

[why]
When the system is going into suspend, set_backlight gets called
after the eDP got blanked. Since smooth brightness is enabled,
the driver will make a call into the DMCU to ramp the brightness.
The DMCU would try to enable ABM to do so. But since the display is
blanked, this ends up causing ABM1_ACE_DBUF_REG_UPDATE_PENDING to
get stuck at 1, which results in a dead lock in the DMCU firmware.

[how]
Disable brightness ramping when the eDP display is blanked.
Signed-off-by: Zi Yu Liao <ziyu.liao@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link.c

@@ -2336,7 +2336,7 @@ bool dc_link_set_backlight_level(const struct dc_link *link,
 			if (core_dc->current_state->res_ctx.pipe_ctx[i].stream) {
 				if (core_dc->current_state->res_ctx.
 						pipe_ctx[i].stream->link
-						== link)
+						== link) {
 					/* DMCU -1 for all controller id values,
 					 * therefore +1 here
 					 */
@@ -2344,6 +2344,13 @@ bool dc_link_set_backlight_level(const struct dc_link *link,
 						core_dc->current_state->
 						res_ctx.pipe_ctx[i].stream_res.tg->inst +
 						1;
+
+					/* Disable brightness ramping when the display is blanked
+					 * as it can hang the DMCU
+					 */
+					if (core_dc->current_state->res_ctx.pipe_ctx[i].plane_state == NULL)
+						frame_ramp = 0;
+				}
 			}
 		}
 		abm->funcs->set_backlight_level_pwm(