drm/msm/mdp5: add missing LM flush bits

For some reason, layer-mixer 3 and 4 were missing.  LM3 is used for
writeback on 8x16.
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/disp/mdp5/mdp5_ctl.c

@@ -468,6 +468,8 @@ u32 mdp_ctl_flush_mask_lm(int lm)
 	case 0:  return MDP5_CTL_FLUSH_LM0;
 	case 1:  return MDP5_CTL_FLUSH_LM1;
 	case 2:  return MDP5_CTL_FLUSH_LM2;
+	case 3:  return MDP5_CTL_FLUSH_LM3;
+	case 4:  return MDP5_CTL_FLUSH_LM4;
 	case 5:  return MDP5_CTL_FLUSH_LM5;
 	default: return 0;
 	}