drm/amd/display: Avoid MPC infinite loop

[why]
In some cases MPC tree bottom pipe ends up point to itself.  This causes
iterating from top to bottom to hang the system in an infinite loop.

[how]
When looping to next MPC bottom pipe, check that the pointer is not same
as current to avoid infinite loop.
Reviewed-by: Josip Pavic <Josip.Pavic@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Aric Cyr <aric.cyr@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/dcn10/dcn10_mpc.c

@@ -49,6 +49,11 @@ void mpc1_set_bg_color(struct mpc *mpc,
 
 	/* find bottommost mpcc. */
 	while (bottommost_mpcc->mpcc_bot) {
+		/* avoid circular linked link */
+		ASSERT(bottommost_mpcc != bottommost_mpcc->mpcc_bot);
+		if (bottommost_mpcc == bottommost_mpcc->mpcc_bot)
+			break;
+
 		bottommost_mpcc = bottommost_mpcc->mpcc_bot;
 	}